noticed at zombie pigman gold/xp farm in multiplayer. There were hundreds of invulnerable zombie pigmen standing in 24 mine carts and not taking entity cramming damage. They were also unable to take damage from the player. They could also not move. This situation was causing thousands of drops and xp to pile up in the wrong places almost crashing the server.

also noticed other frozen invulnerable mobs in the over world.

Since almost all servers have a mob grinder of some sorts, this is a very important issue.