When you bring your vehicle to a mechanic, please clean your nasty shit out first. It ain't gotta be totally perfect, but hell, clean your own nasty junk and bug infestation out first.

Side note: Please make sure to properly disable your car alarm before getting your oil changed. I can guarantee you that the guy working in the pit really appreciates his (/her) eardrums, and the pit is a literal echo chamber.

You want a sledgehammer to the bottom of your transmission? Sure, just let your stupid ass car alarm go off in the shop, go ahead and test your luck with the mechanic in the pit that probably already has a migraine..

Edit: 3rd respect for mechanics. Please God don't sit in your vehicle running the air conditioner while waiting for an oil change, oil gets fucking hot yo! Please let your vehicle cool off, there's a damn fine reason they have an air conditioned waiting room.

There's really only two of them:

1. Don't be stupid.
2. Don't be a dick.

They're not even unspoken, people say them all the time, but some people just don't pay attention I guess.

don't hold your phone while driving

If your seat reclines in an airplane, you have every right to recline it except during takeoff and landing. This one is controversial.

Reading the answers to this questions, I'd say "Not understanding the meaning of most people".

This is probably just a me thing, but if you have to use the restroom, just say that instead of saying what you specifically have to do. I don't need to know any details and you don't need to share them with anyone. The only details worth sharing is if something happens, like a pipe leak/burst or something else serious.