Geroff my horse!
I had Aela (my character's wife) following me when the Battle of Whiterun started. For some reason, she took off running toward Whiterun every time I stopped talking to her. I followed her on my horse for a while, but then she stopped for some reason. I dismounted beside her. She mounted! I could neither get her off nor get her to move.
Map travelling saved me, but I found it amusing anyway.
Who else thinks "mount horse" should be an NPC follower command? (Instead of "kill horse". What a stupid idea, having that as the command.)