Another of my favourites is when cyclists lock bikes to scaffolds on the sidewalk.
I've had to dismantle scaffolds that had bikes locked to them, and I feel sorry for the idiots, but when the scaffold goes, the scaffold goes.
worst of all, every time that was system scaffolding, not tube and clamp. And sometimes I see them locked to ledgers. Any person walking down the street with just a hammer could pop the pins on just that one ledger at the base of the scaffold and take that bike - in 5 seconds.
Granted as opposed to what you see above we're not talking about a tradesman who you assume ought to know better from working in the same environment, but still - common sense should tell you this much.