Topic: Mobs getting stuck.
Can't believe this isn't already up here, but many times when mobs reset to the area around of their spawn points, they start wabbling back and forth and essentially get stuck in place. It doesn't happen every time, but by the time I check out a spawn in a relatively unpopulated area there is usually at least one stuck. It sometimes gets so bad that an entire spawn is stuck on their perches until you kill one.