What is one thing people do that really grinds your gears ?

Situational unawareness, like blocking the entire aisle of a grocery store with your body and your cart while looking at food.

Not lubricating my gears in oil

Using “a” instead of “an”.

Switching up ‘your’ and ‘youre’

Throwing face masks and disposable gloves in the street especially if like my Nan your property backs up to a shopping center car park and people just throw them over the fence into her garden.