The most absurd thing is when you're at Universal and somebody tries to cut in the single rider line so they meet up with their group. I actually called a little girl out on this once. I was like "OK, this is the single rider line. Its supposed to be for single riders. Why should you need to be in line with your family, when there's no expectation that you'll be sitting together, you WILL be split up anyway!" Ultimately the family just got behind me when I refused to cede my position.
Not just line jumping, but big groups using the single rider line is a peeve of mine too. Especially if they try to convince the groupers to let them sit together.