One semester in high school I was the chemistry teacher's 'lab assistant' and did various preparations while she taught a different science section. Most of the labs I have utterly forgotten, but on one occasion I was supposed to prepare a solution of (IIRC) ammonium chloride. The only bottle of the stuff said 'anhydrous', so I poured a bit of the powder into a flask of water. The small explosion produced a big gout of fumes, but I think the flask held together until it hit the floor. No injuries, but a memorable lesson in checking with the teacher when the label doesn't *quite* match the directions.