In my Classical Mechanics class, the professor was fond of giving very long tests, and usually one class period (90 minutes) wasnt long enough to finish them. So, he always scheduled a 3 hour period from 7-10pm on Monday nights when we took our tests, to give everyone enough time to finish. Usually the building was deserted at this time.
One such Monday, about 2 hours into the test, we overhear a conversation from an adjoining (empty) office through the AC vents in the roof. It went something like this:
Guy: "How about in here, I dont think anyone uses this office"
Girl: "Ummm, I dont know.....what if a cleaning lady finds us or something"
Guy: "Everyone went home hours ago, dont worrry about it"
Girl: "Ok, I guess this is alright"
The test is interrupted by snickers from the class (probably 8 guys and 2 girls). But we soon resume working on the test, regardless of the hookup occuring next door. After a few more minutes it starts to get hot and heavy, and they are both moaning and groaning pretty loud. Everyone stops working on their tests and starts to chuckle.
30 seconds later the guy rips an enormous fart. So loud we could hear it through the wall, above all the panting and grunting. The girl's yell of "OH MY GOD YOU JUST FARTED ON ME" are nearly drown out by 10 students and 1 teacher simultaneously falling on the floor laughing in tears.
Needless to say, we got a 30 minute extension on the test because of violent fits of laughter. Definately one of the funniest moments of my entire life.