Hahaha some of these are classic!
We had an exam where there was only one invigilator (due to it being only a few of us doing the exam) and she was quite oldish. This boy raised his hand, and asked her if he could go to the toilet, clearly quite desperate. She ummed and ahhhed for a bit, and then went to the front and got this walkie talkie thing, obviously trying to contact reception. But she just couldn't work out how to turn the thing on! She was pressing random buttons, and everytime it bleeped or made a noise, she would jump about six feet in the air. Everyone in the exam was just watching her, silently laughing, and this boy was wriggling round in his seat.. oh it was so funny. In the end, someone came to the hall to find out why they were receiving two second long walkie talkie messages haha.
I'll also never forget the time the clock stopped in the hall, and the same woman invigilator had to stand on a desk to try and take it down.. but she got stuck, due to her stiff limbs etc, and the other invigilators were all trying to help her get down, everyone was just staring, absolutely captivated!
Aside from that, we've had desks collapsing, the music block (cleverly situated next to the exam hall) having a drum lesson during GCSE Physics, and the English block distributing the books for Lit wrongly, everyone ending up with the wrong novel! Ohhhhh gee times.