My high school chemistry teacher (who had a PhD in Biology -- go figure) would fill a balloon with hydrogen and light it with a match as some sort of experiment (I think she was just bored). This was because the principal made her stop mixing it with oxygen first. Apparently, he became disgusted when cracks started appearing in the windows.

My college chemistry professor decided to make thermite in a flowerpot one day, apparently a common classroom experiment. He turned out the lights for drama, I suppose, and all was going well, until a latecomer wandered into the classroom about two feet from the burning fuse and became confused at the darkness and all the people yelling at him to go away. I think he came within inches of having molten iron splattered on him.