I always find it most curious that (especially in LAX) the walking/talking signposts stand around yelling "US citizens Line 3" and look baffled when asked where the rest of us should be headed, mostly answering by waving their arms in the vague direction of somewhere else.
I've only had one minor run-in there when someone as tall as me (but of significantly greater mass and width) decided to hold the ONLY non-US Resident line and shunt passengers from the FIVE US - Resident lines in front of us.
After 2 minutes, I challenged this marginalisation, only to end up with a nose about 5cm from mine and a glare which indicated that it wasn't a wise move.
Got the line moving, though, and even garnered some scattered applause from the back of our line.
But I do hate it when they ask you the same questions as on the customs form over and over again. Do they SERIOUSLY think that you're going to give a different answer?
And they get VERY annoyed when you say "Yes, exactly as I've declared with my signature on your form"...