The Dude's habit of inspecting food.
This is something I noticed on rewatch. Whenever he picks up food, he looks it over, compares it to the other possibilities, even when he's getting a carton of milk which is just about identical to the other cartons. If he picks a candy or nuts out of a dish, he looks several of them over, picks through several, before deciding on one.
It's just a little character quirk, but I wonder if there was some point to it. I've never heard it brought up by anyone.