Two things that piss me off (financially speaking)
1) Having your doctor's office refer you some other place and they tell you "we already checked with your insurance and they will cover everything." Then a month later you receive a bill for $350 because your insurance rejected the claim.
2) Writing a check for $16.95 to a coworker in April 2005 for your part of a birthday present. In Feb 2006, the check has still not been cashed so you become sick of remembering to add it back into your checking account everytime you receive a statement. Then you receive your March 2006 statement only to see the check went through, but not for $16.95 like you had written it for $16.25. That annoys me more than I can even say