Sunglasses. Unless you wear a really luxe pair I think you expect to lose or break them at some point. So if I misplaced a pair of shades on the bus I'd probably just write it off. Same thing with a hat, probably. Hats are replaceable.
This morning, though, I left my backpack on a bus. It wasn't until the bus was gone and I was grabbing coffee that I realized that. The bag has an address book in it. Also toothbrush and toothpaste, because a lot of days I do dental hygiene at work rathter than at home (not counting brushing my teeth at night.) The real deciding factor was that my partials were in there. They'd be an expensive pain in the ass to replace. So I waited for the bus to come up the other way so I could grab my stuff again.
All of which is a bit embarrassing, but we're getting to the part of the year where I can blame the heat/humidity for breaking my concentration.