Haha, funny story: Summer semester, and a student zips right past me, behind my desk without an 'excuse me', which is a humungoid pet peeve of mine here at work. I call him back, "Excuse me!", and he turns around. Now with this student, I'm always pleasant with; he comes through the front door, and we say to each other. But at that moment, he's the enemy! He went behind enemy territory! So, I'm like:
"Don't do that again."
Stunned, he replies, "Seriously?"
"Well, it was busy and -"
Busy? Yes, there were four or five other students milling around in front of my desk, but what do you do or say to get through a crowd instead of making a quick detour into my work space? You say, 'Excuse me.', right? If this dude had asked first, I would have gladly let him walk behind me, but he didn't and that grinds my gears.