Oh so many:

Never check the chamfer on the piece of steel you just milled with your thumb - Lost tip of finger

Never cycle home after 6 double vodka's - broken shoulder

Never push your mums shopping down the conveyor belt in Tesco's - 3" Scar on right palm

Magnets on TV screens make pretty colours but don't do much for the picture afterwards - New TV

And not forgetting the old favourite, if it looks too high/far/heavy/wide/deep then it probably is.

In my defence most of these were when I was younger, let's hope age brings the much vaunted wisdom

Maybe the best, or worst, was someone I worked with:

When you wear a harness it's better to clip it on rather than fall 75 foot hitting a girder every 6 feet - Broken Back