99.9% of the time when placing a bet the Under will be on the bottom and the Over will be on the top but occasionally for whatever reason a book will have the Over on the bottom and the Under on top. I'm sure it's a mistake by the book but it's nearly gotten me into trouble on several occasions. I usually catch it... but not today I didn't. When I got home today I was happy to see that I had nailed an Under prop bet(or so I thought). When I looked at the graded wager it said that I had bet the Over. I'm 99% certain that I clicked the bottom box because the way I have it written down in my notebook corresponds with my wager and the way it was written on the screen.
Moral of the story... before finalizing your wager make sure that the Over and Under are actually in the correct spot.