So a few years ago in a daycare when my son was 4, another parent yelled at my son. She was upset with her own son who was having a tantrum when I came walking up to pick up mine. He kept asking her little boy if he was ok and she kept telling him he needed to be left alone. I bent down to tell my son we needed to go when she got right in his face and yelled "Get out of his face! You don't want to see what can happen when you get in someone's face!". I told her she needed to get out of my kids face or she needed to have me in her face. The teacher came over and led me away into the back area and she left.
Fast forward to tonight at my sons elementary school curriculum night (hes 6 now) I saw her standing in the hall handing out flyers. I waited until most of the parents has moved on before I approached her. I asked her if she remembered me from the daycare. She said yes with a smile. I asked her if she was on PTA. She said no. I replied, "thank God. I wanted to join the PTA but I wanted to make sure you weren't on it. I remember the situation with you an my son so I'd rather not be around you." turned on my heel and walked away.
Petty? Sure. But it felt great.