When I was younger, poorer, dumber, and full of piss n vinegar (aka Gamble) I watched a cocky "kid" walk into a pool room and offer anyone of us the 7 ball sight unseen. I was probably the weakest of my roomates at the time, plus I was pretty much on tilt kinda drunk.. So I bet everything I had (which wasn't too much, but was alot to me at the time) rent money included, as well as a cue I had custom built for my father (hadn't given it to him yet) etc.. etc. Race to 16 with my roomate playing.
As the kid proceeded to run racks, on top of the general cocky attitude, my roomate just fell apart. (He was a low A player) he ended up missing the 7 ball with ball in hand at one point.)
That was probably the dumbest thing I've ever done in pool. Kid turned out to be John Schmidt, and the final score was 16 : 4, and the race took about as long as one of my average races to 7..
I will say though, I picked up a good line that night from him..

After my roomate was so shaken that he missed the 7 with ball in hand, I called over to John and said "If you wouldn't mind I'd really like to finish this set out, I'm drunk off my ass, and it is my money on the line."
Very cooly (point in fact it was the very thing I remember most about the match) this kid looks over at me and says "Nah, I'm good where I'm at."
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The other dumbest thing I've ever done in pool, was years ago while I was sitting there taking my practice strokes for a 9 ball I said to myself (even though it was a duck) "I'm going to miss this.." and I did. Let me tell you, that phrase will make you miss everytime, as well as turn you into the worlds biggest choker for awhile. Dunny why I said it to myself, but here it is 10 years later and it still pops in my head from time to time and makes me choke out. Now all I'm thinking about on key balls is "Don't say the phrase Dave.. Don't say it!!" instead of thinking about the shot.
DJ