Remember science class? Remember the time the teacher dropped bits of sodium into a bowl of water? We would watch as the element skipped about, fizzing away as it burned. Well, for one teacher, this tidy little demonstration didn't go exactly to plan.

A safety glass screen is usually placed between the bowl and the students, so random bits of sodium don't jump out and scald them. One particular teacher decided that he would put the screen over the bowl, lifting it up to drop in sodium. He did this several times, so all the children could see. When he was done, he removed the screen from the bowl.
2 Na + 2 H2O = 2 NaOH + H2
Sodium in water produces hydrogen gas. And this teacher had the bad judgement to have a Bunsen burner burning near the edge of his desk. When he lifted the glass screen, the accumuated hydrogen exploded.
The students were just leaving the classroom when they heard an almighty BANG! They turned back to see the teacher on his ass looking shell-shocked, with bits of the overhead florescent lighting falling down from the ceiling.
A science teacher really should know better than to let hydrogen build up. But that said, in the same department, another teacher accidentally let a senior student make nitroglycerine. They were picking bits of lab equipment out of the ceiling for a fortnight. This should qualify for an Honorable Mention, since none of the students were hurt (except maybe from laughing) and the science teachers really should have known better; despite their best efforts, they remain in the gene pool.