I started a new world on 15w46a and went exploring in caves for resources.

I came across a skeleton that stopped interacting with the world.

When I approach it from behind or in front, the hostle mob ai does not engage me; i.e. it does nothing.

It took a couple of heart of damage when I swung my sword at it, but every hit thereafter it does not take damage.

If I try to occupy the same space as it, then the entity collision mechanic kicks in and pushes me back.

If I use the in game command /difficulty to switch to peaceful, it does not disappear.

If I mine the block out from under it, the entity does not fall.

If I try to drown it or suffocate it with a couple a pieces of sand dropped from above it does not take any damage.

I cannot place cobblestone blocks in the space that it occupies.

However, if I quit the client to menu and relaunch the world, the skeleton entity has now gone.