Once in school, after school, I needed some oxygen for something. Every day I did oxygen, but on the long way I was looking for a shortcut, so I did the chemical calculation of what connects with what. I took a metal (which has oxygen in it) and threw it into chloric acid with the understanding that it would produce oxygen for me.
No oxygen came out. On the contrary a huge green cloud of chlorine was released and I almost died and also there was a whole class I did not know was left in punishment and almost died from the chlorine. Lucky there was an older man with them who saw green clouds and people starting to suffocate and immediately understood and cleared the building and me.