I was promised a performance bonus. My review was just low enough to deny my bonus. Nobody told me it wouldn't be paid, I found out by going through my paystubs carefully.
When I quit, the company shortchanged me a week's worth of salary. When I sent them an email with all the correct dates, details, etc. they said there had been a mistake and that they would pay me in the next two business days. I got paid, but if I hadn't been paying attention I would be out a week's pay. The company pays employees one week in arrears so you have to be very careful and make sure all the amounts are correct.