Oh, I've got two stories. One was a bomb threat, the other a prankster's doing.
I'll start with the bomb threat.
2 years ago when I was a senior in high school, there was a bomb threat sent out to a school, but because there's more than one school within the district with the same name, both schools were evacuated around the time that this bomb was apparently going to go off.
Naturally, the school doesn't tell us the truth, they just say its a fire drill, to get onto the football field, blah blah. What made it bad was that I had the freakin flu at the time, it was my birthday that day, AND we had to wait outside for an hour in a foot of snow. Oh, I'll never forget that day.
The next story was something that happened last year. I think it was one of my midterm tests, and I remember that it was raining all that day. It was a biology midterm in the mid morning. I had gotten there a bit early to be sure to get a good seat in the gym. I was sitting near the adjacent doors to avoid any crowds, cramming, when the doors slam open and some guy wearing all sorts of layers (including on his face) starts screaming "EXAMS! EXAMS!" And tosses a stack of papers down the hallway. A TA runs out the door and grabs his hood as he heads for the exit stairs, but it rips off, and (buhahaah) he falls down the stairs. The TA just stands there for a minute, holding onto the hood, while every student in that hall pushes the exams away from them with their feet (There's NO way you'd want to get burned for stealing or reading exam papers 15 minutes before the exam, that's just stupid). Then the professor and some more TAs come out, pick up the papers, and we wait another 10 minutes for them to set up.
Ok, exam continues as planned. About one hour in the exam, the fire alarm goes off. So we go outside, in the freezing rain for 20 minutes, then go back in when the say it was just a prank. We get back into the exam, 30 minutes later, the alarm goes off again! So outside we go...again. Getting even more wet and even more cold than before. (grrr...) Still pranky mcprankster is having some fun messing up everyone's exam time. We get back in, 15-20 minutes later ANOTHER fire alarm! At that point, the professor just yells out through the megaphone "Exam's cancelled, just go home now"
Man was I ever mad. Most definitely the worst exam experience...ever.