At line 158 in ChunkRegionLoader there is the code

if (this.b.isEmpty())

{ // Do stuff }

else
ChunkCoordIntPair chunkcoordintpair = (ChunkCoordIntPair) this.b.keySet().iterator().next();

The problem is that 'b' is a concurrent hashmap, and its possible for the map to return false for the isEmpty() check, but the entry is removed which then throws an not found error when it attempts to iterate a few milliseconds later.

This has happened to me, and when it does happen. It kills the entire file saving thread.

I only found out about this because someone complained that their world was no longer saving and they lost all their work.