Not a practical joke but a near miss in a sawmill. I worked in a sawmill that had a great safety record while I was there (probably helped significantly by the fact that I was based in the office and not in the factory), but the manager of the green-mill (where they cut the logs into planks) had been there a long time. He told a story of a saw operator that was sitting in his operators chair (by the time I started there the operators were in separate rooms which were insulated from the noise and the risk, but in those days was out in the mill without a cage) when the large band-saw came off it's drive rollers. This was about 150mm wide and about 4m total length. It flew towards the operator so quickly he had no time to get out of the way. Now this thing had been spinning at whatever RPM as it came off and was razor sharp so had the potential to do serious harm. It wrapped around the operators chair in such a way that the operator could not move from where he was without the engineers getting the oxy-acetylene out and cutting the band-saw away. The only injury the operator ended up with was a small cut to his thumb that didn't even need to be stitched. Apparently he needed a change of trousers though.