Geez, I've done a few pretty dumb things.

Number one has to be leaving a baggie with half an ounce of pot in it on the table, out in the open, all day, while my friend's parents were home. Luckily they didn't see it, but that's pretty damn stupid.

Other than that, just random stupid things: putting things back in the wrong place, shouting at strangers. A friend and I smoked up in a forest and had a freakout because some dude shined a flashlight at us, and we ended up crashing through the brush in the dark because we were too paranoid to stay put.