Program: 17693
Project: 17693
Buggy source:
public boolean generate(World world,Random random,int x,int y,int z){
  int type;
  if (this.mushroomType >= 0) {
    type=this.mushroomType;
  }
 else {
    type=random.nextInt(2);
  }
  int height=random.nextInt(3) + 4;
  boolean flag=true;
  if (y >= 1 && y + height + 1 < 256) {
    int blockID;
    int posY;
    int l1;
    int posX;
    for (blockID=y; blockID <= y + 1 + height; ++blockID) {
      byte b0=3;
      if (blockID <= y + 3) {
        b0=0;
      }
      for (posY=x - b0; posY <= x + b0 && flag; ++posY) {
        for (l1=z - b0; l1 <= z + b0 && flag; ++l1) {
          if (blockID >= 0 && blockID < 256) {
            Block block=world.getBlock(posY,blockID,l1);
            if (posX != 0 && block != null && !block.isLeaves(world,posY,blockID,l1) && posX != NContent.glowshroom) {
              flag=false;
            }
          }
 else {
            flag=false;
          }
        }
      }
    }
    if (!flag) {
      return false;
    }
 else {
      Block blockb=world.getBlock(x,y - 1,z);
      if (blockb != Blocks.mycelium && blockb != Blocks.netherrack && blockb != NContent.taintedSoil && blockb != Blocks.soul_sand) {
        return false;
      }
 else {
        int j2=y + height;
        if (type == 1) {
          j2=y + height - 3;
        }
        for (posY=j2; posY <= y + height; ++posY) {
          l1=1;
          if (posY < y + height) {
            ++l1;
          }
          if (type == 0) {
            l1=3;
          }
          for (posX=x - l1; posX <= x + l1; ++posX) {
            for (int posZ=z - l1; posZ <= z + l1; ++posZ) {
              int meta=5;
              if (posX == x - l1) {
                --meta;
              }
              if (posX == x + l1) {
                ++meta;
              }
              if (posZ == z - l1) {
                meta-=3;
              }
              if (posZ == z + l1) {
                meta+=3;
              }
              if (type == 0 || posY < y + height) {
                if ((posX == x - l1 || posX == x + l1) && (posZ == z - l1 || posZ == z + l1)) {
                  continue;
                }
                if (posX == x - (l1 - 1) && posZ == z - l1) {
                  meta=1;
                }
                if (posX == x - l1 && posZ == z - (l1 - 1)) {
                  meta=1;
                }
                if (posX == x + (l1 - 1) && posZ == z - l1) {
                  meta=3;
                }
                if (posX == x + l1 && posZ == z - (l1 - 1)) {
                  meta=3;
                }
                if (posX == x - (l1 - 1) && posZ == z + l1) {
                  meta=7;
                }
                if (posX == x - l1 && posZ == z + (l1 - 1)) {
                  meta=7;
                }
                if (posX == x + (l1 - 1) && posZ == z + l1) {
                  meta=9;
                }
                if (posX == x + l1 && posZ == z + (l1 - 1)) {
                  meta=9;
                }
              }
              if (meta == 5 && posY < y + height) {
                meta=0;
              }
              Block block=world.getBlock(posX,posY,posZ);
              if ((meta != 0 || y >= y + height - 1) && (block == null || block.canBeReplacedByLeaves(world,posX,posY,posZ))) {
                Block localID=type == 1 ? NContent.glowshroomBlue : NContent.glowshroomGreen;
                world.setBlock(posX,posY,posZ,localID,meta,0);
              }
            }
          }
        }
        for (posY=0; posY < height; ++posY) {
          Block block=world.getBlock(x,y + posY,z);
          if (block == null || block.canBeReplacedByLeaves(world,x,y + posY,z)) {
            Block localID=type == 1 ? NContent.glowshroomBlue : NContent.glowshroomGreen;
            world.setBlock(x,y + posY,z,localID,10,0);
          }
        }
        return true;
      }
    }
  }
 else {
    return false;
  }
}

Fixed version (ground truth):
public boolean generate(World world,Random random,int x,int y,int z){
  int type;
  if (this.mushroomType >= 0) {
    type=this.mushroomType;
  }
 else {
    type=random.nextInt(2);
  }
  int height=random.nextInt(3) + 4;
  boolean flag=true;
  if (y >= 1 && y + height + 1 < 256) {
    int blockID;
    int posY;
    int l1;
    int posX=0;
    for (blockID=y; blockID <= y + 1 + height; ++blockID) {
      byte b0=3;
      if (blockID <= y + 3) {
        b0=0;
      }
      for (posY=x - b0; posY <= x + b0 && flag; ++posY) {
        for (l1=z - b0; l1 <= z + b0 && flag; ++l1) {
          if (blockID >= 0 && blockID < 256) {
            Block block=world.getBlock(posY,blockID,l1);
            if (posX != 0 && block != null && !block.isLeaves(world,posY,blockID,l1) && blockID != Block.getIdFromBlock(NContent.glowshroom)) {
              flag=false;
            }
          }
 else {
            flag=false;
          }
        }
      }
    }
    if (!flag) {
      return false;
    }
 else {
      Block blockb=world.getBlock(x,y - 1,z);
      if (blockb != Blocks.mycelium && blockb != Blocks.netherrack && blockb != NContent.taintedSoil && blockb != Blocks.soul_sand) {
        return false;
      }
 else {
        int j2=y + height;
        if (type == 1) {
          j2=y + height - 3;
        }
        for (posY=j2; posY <= y + height; ++posY) {
          l1=1;
          if (posY < y + height) {
            ++l1;
          }
          if (type == 0) {
            l1=3;
          }
          for (posX=x - l1; posX <= x + l1; ++posX) {
            for (int posZ=z - l1; posZ <= z + l1; ++posZ) {
              int meta=5;
              if (posX == x - l1) {
                --meta;
              }
              if (posX == x + l1) {
                ++meta;
              }
              if (posZ == z - l1) {
                meta-=3;
              }
              if (posZ == z + l1) {
                meta+=3;
              }
              if (type == 0 || posY < y + height) {
                if ((posX == x - l1 || posX == x + l1) && (posZ == z - l1 || posZ == z + l1)) {
                  continue;
                }
                if (posX == x - (l1 - 1) && posZ == z - l1) {
                  meta=1;
                }
                if (posX == x - l1 && posZ == z - (l1 - 1)) {
                  meta=1;
                }
                if (posX == x + (l1 - 1) && posZ == z - l1) {
                  meta=3;
                }
                if (posX == x + l1 && posZ == z - (l1 - 1)) {
                  meta=3;
                }
                if (posX == x - (l1 - 1) && posZ == z + l1) {
                  meta=7;
                }
                if (posX == x - l1 && posZ == z + (l1 - 1)) {
                  meta=7;
                }
                if (posX == x + (l1 - 1) && posZ == z + l1) {
                  meta=9;
                }
                if (posX == x + l1 && posZ == z + (l1 - 1)) {
                  meta=9;
                }
              }
              if (meta == 5 && posY < y + height) {
                meta=0;
              }
              Block block=world.getBlock(posX,posY,posZ);
              if ((meta != 0 || y >= y + height - 1) && (block == null || block.canBeReplacedByLeaves(world,posX,posY,posZ))) {
                Block localID=type == 1 ? NContent.glowshroomBlue : NContent.glowshroomGreen;
                world.setBlock(posX,posY,posZ,localID,meta,0);
              }
            }
          }
        }
        for (posY=0; posY < height; ++posY) {
          Block block=world.getBlock(x,y + posY,z);
          if (block == null || block.canBeReplacedByLeaves(world,x,y + posY,z)) {
            Block localID=type == 1 ? NContent.glowshroomBlue : NContent.glowshroomGreen;
            world.setBlock(x,y + posY,z,localID,10,0);
          }
        }
        return true;
      }
    }
  }
 else {
    return false;
  }
}