Mr. Z, our junior high school science teacher.
He was showing us some chemical reactions one day, including baking soda and vinegar and other high-visibility, low danger reactions. Then he set up some beakers with a couple of solutions, a flask that he heated over a Bunsen burner, and some coiled tubing. Essentially it looked like a still.
He had us all put on safety glasses (which wasn’t at all common in the late 1960s – it just wasn’t part of our reality then) and stand at the very back of the room with our hands in front of our faces. He warned us that “this apparatus could explode”. Then he lit the burner under one of the flasks and started pouring very slowly from the beaker into the flask as it heated. As I recall, he had a limp balloon attached to the top of one of the flasks in the apparatus. The intent was that the balloon would fill from the gas created by the reaction. He himself ducked down behind the solid pedestal table as he poured from the beaker into the flask. He looked very nervous as he did it, and he was very careful as he made the pour.
All of a sudden, and with no warning at all, the flask with the balloon on it actually did “explode”, throwing glass halfway across the room. We, who had been prepped and warned to expect this possibility, were nonplussed, waiting for “what’s next”. Mr. Z emerged from behind his table white as a sheet. He was terribly concerned that the explosion had occurred and afraid that one of us might have been hurt. He was also very shaken by the fact of the explosion because… it should never have happened.
He admitted to us later in the class that all of the dramatics, the warnings, the safety glasses, the “stand at the back of the room and shield yourselves” was all just a buildup to… “the balloon was supposed to inflate, and then I was going to pop it and ‘scare’ everyone”. We saw something that wasn’t supposed to happen, and we weren’t surprised by it because we expected it. He saw something that also wasn’t suppose to happen, but did because of an accident in his own preparation, and scared the piss out of him.
I learned to expect the unexpected, and not to be too comfortable with predictions.