For almost all of the "wandering mobs", there is a bug with pathfinding and water streams:
If a mob is wandering around and gets transported away by a water stream during the process, it will always try to walk back to the block it was originally heading towards (red block in the screenshots), no matter how far away it is.

Notice the similarity to MC-79980, which was fixed in the 15w37a snapshot.

Using the setup shown in the screenshots, I tested and confirmed this for the following mobs:

• Sheep
• Cow
• Mooshroom
• Pig
• Horse
• Creeper
• Zombie
• Zombie Pigman
• Spider
• Cave Spider

Interestingly enough, the following mobs do not walk all the way back, but only a certain number of blocks:

• Skeleton (7 blocks)
• Witch (8 blocks)
• Silverfish (2 blocks)
• Ocelot (between 5 and 12 blocks)
• Blaze (between 11 and 13 blocks)

I can also partially confirm the bug for rabbits: About one in ten walks back to the red marker (with the interesting characteristic of jumping back 3 blocks when reaching it).

I could not confirm the bug for these mobs:

• Wolf (presumably their wiggling animation cancels the pathfinding when leaving the water)
• Chicken

However, both of them exhibit the same behaviour as the other mobs while in water (fighting the current)!

I only tested this in recent snapshots, but it most likely occurs in earlier versions as well.

This bug can cause some very unexpected behaviour in mob farms which rely on the "wandering" mechanic, e.g. by making mobs walk off of a spawn platform.