I consider an Action dumb when it happens for at least the 2nd time. The 1st time I book into "learning".
True dat. Experience is the ability to recognize a mistake when you are about to make it the second time! [yesnod]
As for my stupid moments: my first planetary station landing on a high-g planet. I did my usual nosedive-boost-approach. Got a really nice bounce off the landing pad.
I also learned the hard way that you should not place the "Jettison all" button next to any other one that you would occasionally use.
Another nice episode was when I flew to Maia, to get the last meta alloys I needed for Farseer. On the way back, only 2 or 3 jumps from Deciat, I wanted to scoop some fuel. Noticed too late that the system had 2 planets in very close orbit. I got grilled to medium-well, but made it out alive. Only my cargo scoop malfunctioned while I was frantically trying to escape to cooler waters, leaving a nice poop-trail of meta-alloy canisters in my wake.
So back to Maia it was. This time, I got intercepted on the way back, by another player. Again, I escaped, but again not without cargo scoop incontinence. That was the last time I played in open while running important errands.
Just recently, I set out on my first big exploration mission. Equipped my ship with a module with 2 heat sinks, just in case. Just about 1,000 ly outside the bubble, I accidentally switched my fire groups and popped them both while trying to scan a planet. Thankfully, I never needed them during my trip.
And then there was that one day, when my first ever PvP contact coincided with my first ever "tip off message" run, which left me completely overchallenged. I got out unharmed, but that unfortunate CMDR whose planet-parked ship I mistook for a hostile mission target did not. Turned out later he was a combat logger, so no sleep was lost over that episode. In case you missed it, the full story is
here.