Program: 34119
Project: 34119
Buggy source:
@EventHandler(priority=EventPriority.LOWEST,ignoreCancelled=true) public void onPlayerMove(PlayerMoveEvent e){
  final Player player=e.getPlayer();
  if (player.isDead()) {
    return;
  }
  if (!player.getWorld().getName().equalsIgnoreCase(Settings.worldName)) {
    return;
  }
  if (player.isOp() && !Settings.damageOps) {
    return;
  }
  if (VaultHelper.checkPerm(player,"String_Node_Str") || VaultHelper.checkPerm(player,"String_Node_Str")) {
    return;
  }
  if (player.getGameMode().equals(GameMode.CREATIVE)) {
    return;
  }
  final Location playerLoc=player.getLocation();
  final Block block=playerLoc.getBlock();
  if (Settings.rainDamage > 0D && isRaining) {
    boolean hitByRain=true;
    for (int y=playerLoc.getBlockY(); y < playerLoc.getWorld().getMaxHeight(); y++) {
      if (!playerLoc.getWorld().getBlockAt(playerLoc.getBlockX(),y,playerLoc.getBlockZ()).getType().equals(Material.AIR)) {
        hitByRain=false;
        break;
      }
    }
    if (!hitByRain) {
      wetPlayers.remove(player);
    }
 else {
      boolean acidPotion=false;
      Collection<PotionEffect> activePotions=player.getActivePotionEffects();
      for (      PotionEffect s : activePotions) {
        if (s.getType().equals(PotionEffectType.WATER_BREATHING)) {
          acidPotion=true;
        }
      }
      if (acidPotion) {
        wetPlayers.remove(player);
      }
 else {
        if (!wetPlayers.contains(player)) {
          wetPlayers.add(player);
          new BukkitRunnable(){
            @Override public void run(){
              if (!isRaining || player.isDead()) {
                wetPlayers.remove(player);
                this.cancel();
              }
 else               if (player.getLocation().getWorld().getName().equalsIgnoreCase(Settings.worldName)) {
                Collection<PotionEffect> activePotions=player.getActivePotionEffects();
                for (                PotionEffect s : activePotions) {
                  if (s.getType().equals(PotionEffectType.WATER_BREATHING)) {
                    wetPlayers.remove(player);
                    this.cancel();
                    return;
                  }
                }
                for (int y=player.getLocation().getBlockY(); y < player.getLocation().getWorld().getMaxHeight(); y++) {
                  if (!player.getLocation().getWorld().getBlockAt(player.getLocation().getBlockX(),y,player.getLocation().getBlockZ()).getType().equals(Material.AIR)) {
                    wetPlayers.remove(player);
                    this.cancel();
                    return;
                  }
                }
                if (Settings.rainDamage > 0D) {
                  double health=player.getHealth() - (Settings.rainDamage - Settings.rainDamage * getDamageReduced(player));
                  if (health < 0D) {
                    health=0D;
                  }
 else                   if (health > 20D) {
                    health=20D;
                  }
                  player.setHealth(health);
                  player.getWorld().playSound(playerLoc,Sound.FIZZ,3F,3F);
                }
              }
 else {
                wetPlayers.remove(player);
                this.cancel();
              }
            }
          }
.runTaskTimer(plugin,0L,20L);
        }
      }
    }
  }
  if (!block.isLiquid()) {
    return;
  }
  if (playerLoc.getBlockY() < 1) {
    final Vector v=new Vector(player.getVelocity().getX(),1D,player.getVelocity().getZ());
    player.setVelocity(v);
  }
  if (burningPlayers.contains(player)) {
    return;
  }
  if (Settings.allowSpawnNoAcidWater) {
    if (playerLoc.getBlockY() > Settings.sea_level) {
      if (plugin.getSpawn().isAtSpawn(playerLoc)) {
        return;
      }
    }
  }
  if (block.getType().equals(Material.STATIONARY_WATER) || block.getType().equals(Material.WATER)) {
    Entity playersVehicle=player.getVehicle();
    if (playersVehicle != null) {
      if (playersVehicle.getType().equals(EntityType.BOAT)) {
        return;
      }
    }
    Collection<PotionEffect> activePotions=player.getActivePotionEffects();
    for (    PotionEffect s : activePotions) {
      if (s.getType().equals(PotionEffectType.WATER_BREATHING)) {
        return;
      }
    }
    burningPlayers.add(player);
    new BukkitRunnable(){
      @Override public void run(){
        if (player.isDead()) {
          burningPlayers.remove(player);
          this.cancel();
        }
 else         if (player.getLocation().getBlock().isLiquid() && player.getLocation().getWorld().getName().equalsIgnoreCase(Settings.worldName)) {
          if (!Settings.acidDamageType.isEmpty()) {
            for (            PotionEffectType t : Settings.acidDamageType) {
              if (t.equals(PotionEffectType.BLINDNESS) || t.equals(PotionEffectType.CONFUSION) || t.equals(PotionEffectType.HUNGER)|| t.equals(PotionEffectType.SLOW)|| t.equals(PotionEffectType.SLOW_DIGGING)|| t.equals(PotionEffectType.WEAKNESS)) {
                player.addPotionEffect(new PotionEffect(t,600,1));
              }
 else {
                player.addPotionEffect(new PotionEffect(t,200,1));
              }
            }
          }
          if (Settings.acidDamage > 0D) {
            double health=player.getHealth() - (Settings.acidDamage - Settings.acidDamage * getDamageReduced(player));
            if (health < 0D) {
              health=0D;
            }
 else             if (health > 20D) {
              health=20D;
            }
            player.setHealth(health);
            player.getWorld().playSound(playerLoc,Sound.FIZZ,2F,2F);
          }
        }
 else {
          burningPlayers.remove(player);
          this.cancel();
        }
      }
    }
.runTaskTimer(plugin,0L,20L);
  }
}

Fixed version (ground truth):
@EventHandler(priority=EventPriority.LOWEST,ignoreCancelled=true) public void onPlayerMove(PlayerMoveEvent e){
  final Player player=e.getPlayer();
  if (player.isDead()) {
    return;
  }
  if (!player.getWorld().getName().equalsIgnoreCase(Settings.worldName)) {
    return;
  }
  if (player.isOp() && !Settings.damageOps) {
    return;
  }
  if (VaultHelper.checkPerm(player,"String_Node_Str") || VaultHelper.checkPerm(player,"String_Node_Str")) {
    return;
  }
  if (player.getGameMode().equals(GameMode.CREATIVE)) {
    return;
  }
  final Location playerLoc=player.getLocation();
  final Block block=playerLoc.getBlock();
  if (Settings.rainDamage > 0D && isRaining) {
    boolean hitByRain=true;
    for (int y=playerLoc.getBlockY() + 1; y < playerLoc.getWorld().getMaxHeight(); y++) {
      if (!playerLoc.getWorld().getBlockAt(playerLoc.getBlockX(),y,playerLoc.getBlockZ()).getType().equals(Material.AIR)) {
        hitByRain=false;
        break;
      }
    }
    if (!hitByRain) {
      wetPlayers.remove(player);
    }
 else {
      boolean acidPotion=false;
      Collection<PotionEffect> activePotions=player.getActivePotionEffects();
      for (      PotionEffect s : activePotions) {
        if (s.getType().equals(PotionEffectType.WATER_BREATHING)) {
          acidPotion=true;
        }
      }
      if (acidPotion) {
        wetPlayers.remove(player);
      }
 else {
        if (!wetPlayers.contains(player)) {
          wetPlayers.add(player);
          new BukkitRunnable(){
            @Override public void run(){
              if (!isRaining || player.isDead()) {
                wetPlayers.remove(player);
                this.cancel();
              }
 else               if (player.getLocation().getWorld().getName().equalsIgnoreCase(Settings.worldName)) {
                Collection<PotionEffect> activePotions=player.getActivePotionEffects();
                for (                PotionEffect s : activePotions) {
                  if (s.getType().equals(PotionEffectType.WATER_BREATHING)) {
                    wetPlayers.remove(player);
                    this.cancel();
                    return;
                  }
                }
                for (int y=player.getLocation().getBlockY() + 1; y < player.getLocation().getWorld().getMaxHeight(); y++) {
                  if (!player.getLocation().getWorld().getBlockAt(player.getLocation().getBlockX(),y,player.getLocation().getBlockZ()).getType().equals(Material.AIR)) {
                    wetPlayers.remove(player);
                    this.cancel();
                    return;
                  }
                }
                if (Settings.rainDamage > 0D) {
                  double health=player.getHealth() - (Settings.rainDamage - Settings.rainDamage * getDamageReduced(player));
                  if (health < 0D) {
                    health=0D;
                  }
 else                   if (health > 20D) {
                    health=20D;
                  }
                  player.setHealth(health);
                  player.getWorld().playSound(playerLoc,Sound.FIZZ,3F,3F);
                }
              }
 else {
                wetPlayers.remove(player);
                this.cancel();
              }
            }
          }
.runTaskTimer(plugin,0L,20L);
        }
      }
    }
  }
  if (!block.isLiquid()) {
    return;
  }
  if (playerLoc.getBlockY() < 1) {
    final Vector v=new Vector(player.getVelocity().getX(),1D,player.getVelocity().getZ());
    player.setVelocity(v);
  }
  if (burningPlayers.contains(player)) {
    return;
  }
  if (Settings.allowSpawnNoAcidWater) {
    if (playerLoc.getBlockY() > Settings.sea_level) {
      if (plugin.getSpawn().isAtSpawn(playerLoc)) {
        return;
      }
    }
  }
  if (block.getType().equals(Material.STATIONARY_WATER) || block.getType().equals(Material.WATER)) {
    Entity playersVehicle=player.getVehicle();
    if (playersVehicle != null) {
      if (playersVehicle.getType().equals(EntityType.BOAT)) {
        return;
      }
    }
    Collection<PotionEffect> activePotions=player.getActivePotionEffects();
    for (    PotionEffect s : activePotions) {
      if (s.getType().equals(PotionEffectType.WATER_BREATHING)) {
        return;
      }
    }
    burningPlayers.add(player);
    new BukkitRunnable(){
      @Override public void run(){
        if (player.isDead()) {
          burningPlayers.remove(player);
          this.cancel();
        }
 else         if (player.getLocation().getBlock().isLiquid() && player.getLocation().getWorld().getName().equalsIgnoreCase(Settings.worldName)) {
          if (!Settings.acidDamageType.isEmpty()) {
            for (            PotionEffectType t : Settings.acidDamageType) {
              if (t.equals(PotionEffectType.BLINDNESS) || t.equals(PotionEffectType.CONFUSION) || t.equals(PotionEffectType.HUNGER)|| t.equals(PotionEffectType.SLOW)|| t.equals(PotionEffectType.SLOW_DIGGING)|| t.equals(PotionEffectType.WEAKNESS)) {
                player.addPotionEffect(new PotionEffect(t,600,1));
              }
 else {
                player.addPotionEffect(new PotionEffect(t,200,1));
              }
            }
          }
          if (Settings.acidDamage > 0D) {
            double health=player.getHealth() - (Settings.acidDamage - Settings.acidDamage * getDamageReduced(player));
            if (health < 0D) {
              health=0D;
            }
 else             if (health > 20D) {
              health=20D;
            }
            player.setHealth(health);
            player.getWorld().playSound(playerLoc,Sound.FIZZ,2F,2F);
          }
        }
 else {
          burningPlayers.remove(player);
          this.cancel();
        }
      }
    }
.runTaskTimer(plugin,0L,20L);
  }
}