I've done heaps of dumb things.
First dumb thing was not diving from 2005 (when I did two try dives) until 2008 (when I got certified).
Second dumb thing was wasting a lot of money on gear that was not suitable long term for me and vastly vastly overpriced (hadn't discovered nice LDSs and Leisurepro at that stage) and also training that was unnecessary. I have replaced all of it other than my wetsuit (and that's only because it's not crap enough to warrant a new wetsuit, but it is still crap).
Then there are the dives I did early on when I didn't know much and therefore how badly things could go - diving with people I was not comfortable with out of politeness (few OOAs from them, or they have corked, or done something stupid and just generally spoiling my dives), accidentally going into deco once, doing a solo dive very unprepared early on, getting lost because my navigation was crap, things like that.
Then there are all the other silly things I've done in regards to gear such as -accidentally throwing my compass overboard on a dive where I was being assessed on my dive leading ability and thus was responsible for navigation :blush: I think as I hate navigation so much it must have been my subconscious doing that
-wearing wetsuit weights instead of drysuit weight and not realising until the next day (the lightbulb went off
) after a very uncomfortable dive where I was very light
-forgetting to turn a stage bottle on and switching to it on a course
-bunch of times forgetting to hook up my drysuit
-jumping in with gas only turned on slightly (the dive was so shallow I didn't realise until I was taking apart my gear after)
-accidentally setting my computer to 100% O2 (it goes from 21% to 100% as you cycle through and I went one too many) and wondering why it was having a panic attack a few minutes into the dive
-setting up my wing the wrong way around on the backplate after putting it back together
It's a fairly long list... :blush: I'm pretty lucky in that I haven't forgotten critical dive gear, that seems to be the thing I see most from people. The most notable thing that I have forgot was forgetting my wetsuit boots once. Sneakers are a poor replacement but nevertheless I got a decent length dive in, mostly on my head as they were so floaty!