For example, you assume that guy trying to cut the line is a horrible person and a megalomaniac because you've seen this like a thousand times. He really may be that, or maybe he's having an extraordinarily stressful day, or maybe he's just not integrated with the values of your society ("cutting the line is bad, no matter what") or anything else BUT none of all that really helps you think clearly. You just get angry and maybe raise your voice when you're warning him, because "you know" he won't understand otherwise. So you left your values now too because you are busy fighting a stereotype.
IMHO, correct course of action is assuming good faith even with bad actions, and even with persistent bad actions, and thinking about the productive things you can do to change the outcome, or decide that you cannot do anything.
You can perhaps warn the guy, and then if he ignores you, you can even go to security or pick another hill to die on.
I'm not saying that I can do this myself. I fail a lot, especially when driving. It doesn't mean I'm not working on it.