Yes; I babysit for a family weekly. The kids are 4 and 2 and a half. Last week, the 4 year old had a pool party, and the mom asked a friend and I to come and help supervise the kids in the pool. We were warned that all the kids were under 5, and that there were 20 of them, so we would really be doing a lot of work keeping up with them.
All the parents were there, but one little boy's grandfather wasn't really paying attention to him. The little boy reached over (totally unprovoked) and grabbed the 4 year old I babysit by wrapping his hands around his neck and started shaking him. I freaked out and got his hands off and pulled the kid I babysit away, and told the other boy he wasn't allowed to do that. He got mad, and ran out of the hot tub to his grandfather, who turned to give me a nasty look.
I would have loved to smack him upside the head, but I didn't. I later found out from the mom I babysit for that the child has no rules, disrespects any authority figure, and gets away with everything. The family had never RSVP'd, so she hadn't had a chance to warn my friend and I how bad he is.