Just a quick question
I have a couple of answers. If someone interrupts to say hello and then keeps moving, I'm OK with that.
If it's the boss interrupting and he wants to join the conversation for a sec., I may not be OK with it, but I'll deal.
If it's a buddy - a friend to BOTH parties - who interrupts and wants to join the conversation, I don't mind.
Otherwise, I think all other circumstances may be out of order - like when someone interrupts you mid sentence and begins an entirely new and lengthy conversation with the person you were talking to, and later walks away without so much as a word of acknowledgement to you.