Just remembered another stupid.
I was trying to get a pedal off an old Peugeot. It was one of those pedals with a hex socket in the pedal axle.
Well, you know how the left hand pedal is left hand thread. Yeah, I know that too. Clockwise to remove it. So I put the hex key in and turned clockwise. Nothing. I added some leverage, I forget how, heaved, and managed to turn the hex key into a barber's pole. Then I realised...
... I was looking at the pedal from the wrong side. I was looking at it from the inside - so I could see to insert the hex key. Clockwise, looking from the outside (pedal side) of the crank is anticlockwise looking at it from the inside.
Doh!