I've had a few little "moments" that will never leave the shed but the big "doh" mechanical moment for me was when I bought a complete bike off a bloke from the US and after giving everything a careful once over, adjust, lube, check and check again to convince myself that it was safe to take for a blast I did just that.. it was mid blast and on a fast loose gravel decent I discovered that the goofy Americans run the front brake to the left hand! How I stayed upright I have no idea.
:facepalm:
:facepalm: