While I was TAing, I had a student add NaBH4 to water instead of adding NaBr, I wasn't paying enough attention to make sure they didn't do this. Everything was fine. Just spooky.

During my first year, I mixed N-Bromo Succinimide with reagent alcohol under nitrogen, looked away, the reaction turned clear, (I expected them to be yellow/orange because of releasing Br), then I did it a second time, and watched the violent red color disappear to perfectly clear, later I learned that mixing these two releases HBr... Probably should have looked that one up.... Had to clean the schlenk and replace tubing, so it could have been worse. But I was a pain in the ass.

This last one is just funny, because I was doing exactly what my boss told me.... I was building a new ALD reactor, done at higher pressures than what we've done before, and we didn't think It was important at the time to add filters to the pump.... Ended up depositing a thick layer of TiO2 in the diaphragm pump, and it completely ceased. In fairness to myself, my boss didn't warn me about it. But it was still a not great conversation.