Police. See #1. You look at them one time. If you double take or look too long you might attract their attention. In doing so, with a cop with nothing better to do and if in their minds they suspect an ounce of fishy shit going on with you, expect to get followed for a bit so they can check your plates, pull you over and/or get talked to to see what's up with you.
From the movie Street Kings
Coates: Man, we got the one time up in here. Motherfucking po-po. Is there any square cops out there? Or is everybody out for theirs?