Skeletons just stand there. Ever since a few snapshots ago I've noticed that some skeletons (not every one of them) will do absolutely nothing. It doesn't seem to effect any other monster, passive or otherwise. I can hit them once and they don't burn in the sun. Reloading the chunk or the world doesn't fix either.

It also doesn't seem to be superflat worlds only (Normal sized, Amplified, Large biomes, etc) I've seen it happen in all of them.