This is a fun thread! I do bonehead things every day I brew... actually... every day I get out of bed... I'm glad to be in good company.
Mostly it's common things like spraying, dumping, or siphoning hot/cold water, wort, beer all over myself, my kitchen, my significant other.
One memorable day, even more newbie than I am now, I was kegging and extraextra strong company bitter. On someone's advice, I had gone "extra strong" in stages: letting it ferment, adding some DME in a litre of water, and so on. I had started with enough liquid to fill one keg, but the reality of the accumulative math involved didn't really strike home until I was siphoning to my keg and about to overflow, with no spare bottles for the extra beer. Thinking quickly (?!) (not wanting to loose the siphon, not having a "pincher" on my line...) I continued to siphon with one hand while I opened the fridge and grabbed a cold pint glass... and another... and another... and then drained the remaining beer from the secondary by sitting on the floor with the siphon in my mouth. I hate to let good things go to waste, see. Now I have a number of pints of almost flat (but truthfully fairly pleasant) beer sitting in glasses... well... waste not want not...
Anyway, my girlfriend phoned me a little later in the evening (okay... it was about 4pm)(I think I was laying on the couch singing sweetly to myself and watching the world - nay, the room - turn) and asked if she should pick me up to go workout. Obviously, I lied. "Wellll.....you knowwww.... I think.... I think... I'll just stay here and do some homework tonight..."