I have to add a comment because I had this problem today. If your UPDATE fails and you are stumped as to why, consider that there might be a deadlock on the record. I had a problem today when a stored procedure faled to do its job and no-one knew why. I then ran one UPDATE sql from the proc on its own, and got a deadlock error. It turned out that there was a trigger on one of the tables, and because the proc only commits at the end of the code, the UPDATE deadlocked with the trigger UPDATE. This can also happen if another database user is editing the same record via a database editor and hasn't commited his changes yet. The good news is that this can't happen in a web environment.
Anyway, all my tasks are completed, and it's 'min dae' till the project has to be in, so it's to get involved! See you guys in the prac forum!