This happened to me once in college. 

I set my alarm, fell asleep. Then my roommate woke me up saying, "Hey man, your alarm has been going off for awhile now."

I looked at the clock, it read 7:50 and I had an exam at 8. So I throw on some pajama bottoms and run all the way to the exam room, only to find that it was empty. The exam took place the day before. Yeah.