Mine is that although I'm generally a live and let live kind of person, I think it is perfectly acceptable to call people out on car seat safety stuff. Even strangers. I know that makes me pretty much the most annoying person ever but it gets me sooo badly when I see chest clips on a poor innocent babies belly!
If me being a jerk and saying something about it saves your kids little internal organs in the event of an accident, so be it. I'll take one for the team and be the jerk of the day.