As long as we're talking about interactions...
A long-standing peeve of mine: People who don't know how to carry on a decent conversation. Those who've met me will never label me a scintillating conversationalist but there are people that don't even seem to get the basic ebb and flow concept.
These are people that, upon a question from you, will gladly talk about themselves for forty-five minutes without pausing to see if you're paying attention (or still alive) but once they finally wind down, will just sit there, basting in their own juices, rather than learn anything about you.
You ever find yourself next to these people at a dinner party? It's brutal. You ask Wally a pleasant, polite question and suddenly it's "The Wally Show" and there's no off switch.
So, fine. you now know way more than you wanted to about Wally. Okay, you asked. Fair enough. But then Wally sits and stares.
Wally doesn't know anything about you and can't be bothered to ask. No "What do you do?" No "Tried the cheese dip?" No "Single or multiple?" Nothing.
A variant of this but no less irritating, are the people that can't be bothered to respond with more than a one word answer, much less show a polite interest in you.
Me: So, how do you know Sally?
Dick: *stares into middle distance*
Me: Do you like the company?
Dick: Eh. *stares into middle distance*
Me: *stabs self in eye*
How is this acceptable?