Reported by RealM:
https://pastebin.com/caLh7ujT

net.minecraft.world.World.func_175625_s is getTileEntity(BlockPos pos)
net.minecraft.world.World.func_189508_F is getPendingTileEntityAt(BlockPos pos)

As I remember he said that he entered a dungeon

@TheRealM18 1) did you running around whne this happened? 2) was a extern cooler around you when this happend? 3) did this happen when joining the world ?

then it is hopefully a chunkloading problem (the other block was on the edge of a chunk)