Usually if I set an early or important alarm I'll end up either not be able to fall asleep until 3am, wake up at least an hour before the alarm goes off (thereby ensuring I have no time to actually sleep), or both, because fuck you body clock. This happened the night before me and my friends were going to catch the coach for a London day trip the next day. Since we wanted to make a day of it, we had to leave Bristol around 7 in the morning, which meant getting up at half 5/6 in the morning to get ready and make the walk to the bus stop. Naturally my body decides it would be a stellar plan to keep me up until 1 and wake me up at 4. I finally understood why all those Elder gods got their nark on after being awoken. I was less than impressed that morning, but I ended up feeling fairly alright throughout the day.
One of my worst alarm-related experiences was a couple years back in my first year of uni. The morning before my very last exam (I think it was Dynamics) I woke up to my alarm only to fall straight back asleep until 45 minutes before exam start, when I woke up to about 15 messages from about 4 people asking where the fuck I was. Bearing in mind I lived a 30 minute walk from campus and the exam was being held on the far side, another 5 or so minutes, and had to be ready to got into town straight after. I had 5 minutes to get as ready as humanly possible, so I had to choose between breakfast and a shower. Due to the fact that I was going out with friends straight after, I chose the shower since I could just eat after the exam. After the world's fastest shower I peg it to the bus stop to find traffic at a crawl and no bus in sight. I ran the 1 and a half miles from my flat to uni, in the rain, and made it in no more than 5 minutes before the start, panting, wheezing and wanting to throw up due to the lack of food and the impromptu 1.5mi run. I ended up getting 70% on that exam, which was my highest exam result that year.
If I'm honest, I can't really blame the alarm for that. Shouldn't have to bed so late.