Parents that let their children run wild without consideration for others.
My 3 yr old is a big guy and is sometimes mistaken for being older but that's no excuse at all for other kids on the playground screaming at him to run, as though he should join their game, and then ploy right into him in an effort to push him along. It doesn't matter how many times I tell them to leave off the pushing, unless their parents grow a brain and some consideration, the stupid brats aren't going to stop.
Or how about the stupid kids that run around the grocery store, pushing and shoving one another, running up and down the isles, throwing stuff. Or how about the little brats downstairs from me that are left to run wild - tossing things off the balcony, leaving all their toys by the building entrance, playing in the parking lot without regard for people that would like to pull their cars out and leave.
I have no tolerance for people that don't drive with regard for other people's safety either. Use a turn signal people! Don't weave in and out of traffic, most especially when you are speeding! Don't even count on being able to stop on a dime - slow down and stop safely!