Steps to reproduce:

1. Create a new world.
2. Summon a mob, or use /entitydata on an existing one so "NoAI" NBT tag is set to 1.
3. Use /tp with relative coordinates to make it face a different direction than the default.
4. Leave the world and open it back
5. The mobs should be rotated to a set direction... It fixes to the previous position if you use "/tp @e[r=1] ~ ~ ~ ~ ~" on them (using the selectors you want of course)

PS: If you can't reproduce it first time, maybe you just left it on the same direction it gets rotated because of the bug.