Oh ho ho...where to begin... Well the most dangerous thing I ever did was make a flame thrower out of a petrol/plastic mix, two empty mountain dew bottles(that were full prior to the conception of this idea) adhesive bonding putty, some pvc piping and a set of butterfly valves and a bike pump. That thing was amazing! Incinerated a plastic garden gnome before I realised I forgot a heat sink at the muzzle and the thing caught fire. Oh, there was about half a reel of duct tape on it too.
Number two. Not a crazy build but dangerous non the less, spent an hour drawing patterns on the ground in lighter fluid, rigged a match shooter (not heard of one, cool little build to flick and light matches). That one was just for fun. Good fun.
Finally, set my head on fire. Well...my hat, that was a nasty run in with a leaky storm lantern and an open flame. Drippy fire.
This was about when I was 14-15 and is among the top 3 reasons I was banned from metal shop. Oh, speaking of school experiences, I just remembered the day I heated a full pot of paraffin wax on a bunsen burner. Happy days.