Mine is when I've got to get the monthly report to my boss by the last working day of the month and ONE person ALWAYS forgets to get their portion to me.  This has been going on for 2 1/2 years now, and the same person still forgets.  That means that I've completed over 24 reports in this manner, and 24 times he forgets when it's due.  I even send out reminders to all 8 engineers every month when the due date nears, but because he forgets and I have to remind him AGAIN when it isn't received, that means I have to edit his LATE report before I add it to the others in the compilation, and that makes my report late, too.  

Movinonup