Mobs, if crammed in one spot (with maxEntityCramming set to default), will push each other into blocks, which allow them to glitch through them on world load (possibly chunk load).
Not that easy to replicate. Happens 1 in N times, where N is a relatively low integer number.
Attached - 1. before log-out 2. after log-in.
Singleplayer world.
(possibly may be connected with the new entity collisions behaviour with 16w38a)

What is different with other similar bug reports is - mobs seem to be in blocks client side only. Relog puts them back into position. Mobs cannot see the player, even it it appears to be outside of the trap, but player can hit them.