I have a hard time thinking of these because usually I end up unconscious when im doing something stupid and don't remember anything.
However. Testing to see if a new paintball gun worked to expectations was stupid. Now that in itself isn't stupid, it's the fact that I tested on myself. With my solar plexus.
Another one was when some friends and I invented trampoline wrestling. No holds barred, totally unprotected greco-roman wrestling on a bouncy trampoline. Being the paragon of manliness I am I bounced/threw a friend off during a match clean into his fence. I felt like a Spartan after that feat, even though it knocked the air/consciousness out of my friend.