Program: 34781
Project: 34781
Buggy source:
@SubscribeEvent public void onLivingDrops(LivingDropsEvent e){
  if (e.entity.worldObj.isRemote || !e.recentlyHit)   return;
  ItemStack is=null;
  Random rand=e.entityLiving.getRNG();
  if (rand.nextInt(Math.max(1,40 - e.lootingLevel)) == 0 && e.entity instanceof EntityEnderman) {
    is=new ItemStack(ItemList.enderman_head);
    for (    EntityPlayer observer : ObservationUtil.getAllObservers(e.entityLiving,8D))     KnowledgeRegistrations.ENDERMAN_HEAD.tryUnlockFragment(observer,1F);
  }
 else   if (rand.nextInt(14 - Math.min(e.lootingLevel,4)) == 0 && e.entity instanceof EntitySilverfish) {
    boolean drop=rand.nextInt(4) == 0;
    boolean isPlayer=e.source.getEntity() instanceof EntityPlayer;
    if (!drop && isPlayer) {
      ItemStack held=((EntityPlayer)e.source.getEntity()).inventory.getCurrentItem();
      if (held != null && held.getItem() == Items.golden_sword)       drop=true;
    }
    if (drop) {
      is=new ItemStack(ItemList.silverfish_blood);
      for (      EntityPlayer observer : ObservationUtil.getAllObservers(e.entity,8D)) {
        if (KnowledgeRegistrations.SILVERFISH_BLOOD.tryUnlockFragment(observer,0.8F) == UnlockResult.NOTHING_TO_UNLOCK && !KnowledgeRegistrations.INFESTATION_REMEDY.tryUnlockFragment(observer,0.5F).stopTrying) {
          KnowledgeRegistrations.ENHANCED_BREWING_STAND.tryUnlockFragment(observer,0.4F,new byte[]{4});
        }
      }
    }
 else     if (isPlayer && KnowledgeRegistrations.SILVERFISH_BLOOD.tryUnlockFragment((EntityPlayer)e.source.getEntity(),0.65F) == UnlockResult.NOTHING_TO_UNLOCK) {
      EntityPlayer player=(EntityPlayer)e.source.getEntity();
      if (!KnowledgeRegistrations.INFESTATION_REMEDY.tryUnlockFragment(player,0.5F).stopTrying) {
        KnowledgeRegistrations.ENHANCED_BREWING_STAND.tryUnlockFragment(player,0.2F,new byte[]{4});
      }
    }
  }
 else   if (e.entity.dimension == 1 && rand.nextInt(100) == 0)   is=new ItemStack(ItemList.essence,1,EssenceType.SPECTRAL.getItemDamage());
  if (is != null) {
    EntityItem item=new EntityItem(e.entity.worldObj,e.entity.posX,e.entity.posY,e.entity.posZ,is);
    item.delayBeforeCanPickup=10;
    e.drops.add(item);
  }
}

Fixed version (ground truth):
@SubscribeEvent public void onLivingDrops(LivingDropsEvent e){
  if (e.entity.worldObj.isRemote || !e.recentlyHit)   return;
  ItemStack is=null;
  Random rand=e.entityLiving.getRNG();
  if (rand.nextInt(Math.max(1,40 - e.lootingLevel)) == 0 && e.entity instanceof EntityEnderman) {
    is=new ItemStack(ItemList.enderman_head);
    for (    EntityPlayer observer : ObservationUtil.getAllObservers(e.entityLiving,8D))     KnowledgeRegistrations.ENDERMAN_HEAD.tryUnlockFragment(observer,1F);
  }
 else   if (rand.nextInt(14 - Math.min(e.lootingLevel,4)) == 0 && e.entity instanceof EntitySilverfish) {
    boolean drop=rand.nextInt(4) == 0;
    boolean isPlayer=e.source.getEntity() instanceof EntityPlayer;
    if (!drop && isPlayer) {
      ItemStack held=((EntityPlayer)e.source.getEntity()).inventory.getCurrentItem();
      if (held != null && held.getItem() == Items.golden_sword)       drop=true;
    }
    if (drop) {
      is=new ItemStack(ItemList.silverfish_blood);
      for (      EntityPlayer observer : ObservationUtil.getAllObservers(e.entity,8D)) {
        if (KnowledgeRegistrations.SILVERFISH_BLOOD.tryUnlockFragment(observer,0.8F) == UnlockResult.NOTHING_TO_UNLOCK && !KnowledgeRegistrations.INFESTATION_REMEDY.tryUnlockFragment(observer,0.5F).stopTrying) {
          KnowledgeRegistrations.ENHANCED_BREWING_STAND.tryUnlockFragment(observer,0.4F,new byte[]{4});
        }
      }
    }
 else     if (isPlayer && KnowledgeRegistrations.SILVERFISH_BLOOD.tryUnlockFragment((EntityPlayer)e.source.getEntity(),0.65F) == UnlockResult.NOTHING_TO_UNLOCK) {
      EntityPlayer player=(EntityPlayer)e.source.getEntity();
      if (!KnowledgeRegistrations.INFESTATION_REMEDY.tryUnlockFragment(player,0.5F).stopTrying) {
        KnowledgeRegistrations.ENHANCED_BREWING_STAND.tryUnlockFragment(player,0.2F,new byte[]{4});
      }
    }
  }
  if (is != null) {
    EntityItem item=new EntityItem(e.entity.worldObj,e.entity.posX,e.entity.posY,e.entity.posZ,is);
    item.delayBeforeCanPickup=10;
    e.drops.add(item);
  }
}