My high schol chemistry class had a student who was accident prone. Unfortunatly he was across the table from me which was not a good place for me to be. He also had the habit of using more of the ingredients than the instructions called for. I remember once we were making iodine in a beaker, and directing it on a glass container full of ice to condense the iodine. Instead of using a glass tube for this, he used a solid rod. The pressure built up in his beaker until it blew the cork out and iodine gas came pouring out. A passer by would have seen the whole class leaning out the open windows with purple gas billowing out above us. I've never been bothered with goiter since.