Do you have a buddy named Dan? I have a few, and one Dan in particular is the type of guy you'd never want to get caught sitting with at school.
You can't really put your finger on why it's bad to sit with Dan, but doing so would have placed the stigma of Dan upon you which, as this Pie Comic by John McNamee shows, would have been a rookie mistake.
Now that we're all grown up it's fine to sit with Dan, unless he's been drinking all day...
-Via Geeks Are Sexy