In the gym, most of the women use weights too light for themselves, and most of the men too heavy and sloppy. I have only one story to tell that pisses me off about a guy on a bench, making me wait 45 minutes for it, being rude, etc, which was enough to make sure that I only went there when it was not packed, because it wastes my time and 'ruins' my workout.
Unless the person is an obvious rookie and looks like they probably wont mind advice, I usually just do my best to ignore them. No point in sitting there laughing, your there to improve and compete against yourself, not someone else, so I just try to keep my own mind on the job and hand.
I'm also into cars, and various other things, and on so many message boards etc you come across people who don't know much, but obviously think they do, its annoying but thats just how life is. Give someone a little knowlege, and they can be dangerous with it, to themselves or others.
Ignorance breeds the same, tis life.