How about a nice community wiki we can learn from? Please post an example of user behavior that caught you off-guard and forced you to redesign an interface. One example per answer please!
On a PHP/HTML form for recording game plays, the user had to select the game module from a dropdown and then enter the scenario number in your standard input box. After 3 weeks of users painlessly adding records I had a veteran gamer email me to say that no matter what he put in, the form told him the scenario number was invalid.
Cut to the chase - I coded the form to always start with a
# inside that scenario number input. The first 50 users understood to delete that # when entering their 3 digit scenario number, but No. 51 kept trying to submit things like
#37 and my diligent validation script kept rejecting him.
Needless to say, I removed the