When you're with a friend, and they bump into someone they know!
Usually, when the person you're with runs into someone they know, they immediately start talking about things you have no fucking idea about and can add zero input to.
It’s always about people and things you don’t know like, "Have you talked to Tom lately?" or "Did you hear that Anna just came back from Russia?" You never have any idea of the people, or the subject matter that they're talking about, so you just zone out on your phone hoping for the conversation to quickly end. It’s like you're suddenly Patrick Swayze in Ghost. The only reassurance that you sometimes get confirming your physical state, is a brief introduction where you say hi, state your name and then fade back into the shadows to allow the A and B conversation to continue. The situation feels like you're some random asshole, who just weirdly wondered up to two people in a conversation, then just stood there watching them talk.
The fucked up part of the whole friend bump-in is when the person who your friend knows, also has a friend that's like you, who patiently waits like an obedient dog until their owner is done talking. Sometimes the two of you will strangely make eye contact and there is an immediate mutual understanding that neither of you want to talk to each other. You'd both rather wait in silence until your friends are done, and then never see each other ever again.