People who will talk to you (whether at work or school or waiting while your DCs have a lesson) only if you're the only other person there. If there is someone else there they then don't even bother to acknowledge your existence with so much as a nod or a simple 'hello'. Or when their friend arrives it's like you literally don't exist anymore?
AIBU to think these people are a bit rude? And it's a good way to know they really aren't interested in knowing you?