There are two things that will ruin a trip to any park even outside Disney.
At Epcot just as I was going over a bridge and heading to Japan there was a humugous herd of people all dressed alike and they owned the park. I yelled in Spanish some things since i had been warned about Brazilians. Them young people were flying out of the way of a fat lady in an ECV yelling is a language similar to their own. These groups from all over the world literally will plow threw your group, run over anything in their way and will do anything to make their life easier at other peoples' expense.
STROLLER, I HATE STROLLERS. How about the father who whipped past me with a kid on his shoulders and was pushing a stroller in DCA. He stops suddenly in front of me and turns in my face and yells right through me to his family that he had left behind.
They leave strollers in door ways, aisles and walk ways. They mow down people and do not seem to care that if a 300 pound man or woman falls backward onto a stroller then the parent will have a tortilla instead of a kid. It hurts when you hit the ground but they do not seem to care. The kids do not want to be rolled through the clothes and get wopped in the face by packages.
My worst parent story is at the tram. I had a Japanese couple and their little daughter right on the line of the tram at DLR. Behind me was a man pushing on me with all his might, he was leaning on me. He had between his chest and my back his little kid who was getting pounded. I yelled in spanish and english to stop pushing but he would not listen. If I had let in to him then I would have shoved the Japanese family in front of a tram. Oh no I smell a third pet peeve coming on, parents who do not treat kids right.