Got a couple.
First one cycling related. Traffic backed up for road works one afternoon. I slide up the left side of the que, unclip, stop and commence perving on the ridiculously attractive stop/go girl

. As she seductively (may be my imagination) turns the sign from stop to slow, I clip in and start pedalling. While turning right and as I'm half way through saying "how you doing?" I feel the bike straighten while my upper body (and momentum) are travelling right. Next thing I know I'm doing the running man across the road in front of 20 motorists, 1 lolli pop girl and 10 or so road workers. With bike and drink bottles strewn all over the road I quickly realised that the road crew had excavated my side of the road down approx 4 inches creating a step up which straightend my front wheel up and threw me off

.
Second one was many years ago whilst in my second year as an apprentice Truck Mechanic. We had a large furniture removal truck towed in overnight with a crunchy sounding gear box. When a truck is towed they usually drop the tailshaft off at the diff to prevent driveline failures. My first job was to bring the truck up to the workshop and start diagnosing the gearbox noise. Trying to be a little too clever I decided to refit the tailshaft in the workshop instead of rolling around in the dirt and gravel where the truck had been dropped. Even cleverer was the fact that I would probably have to drop it again anyway if the box had to come out. So after lugging this 80kg lump of steel 30 metres up to the workshop, I return to the truck, start the engine, select first gear and drop the clutch. Nothing. Say What? Gearbox must really be cactus. Then it hits me

. So I sheepishly return to the workshop. Stand next to the tailshaft and look around to see if anyone has realised what I've done. Coast looks clear. I bend down and as I stand up with the tailshaft in my arms the entire work shop crew jumps out from behind their jobs and give me a standing ovation

. Man did I feel dumb. Felt a bit better a few years later when I watched a Mechanic with many more years experience than myself do the exact same thing at a different work place. I was even kind enough to offer him a hand carrying the tailshaft to the workshop

(but not back again

).