The social problem
of introducing rather different groups of friends to each other. Sometimes it works well, and other times you get parties with two groups of people on opposite sides of the room, bitching
about the musical tastes of the other group. It makes me somewhat hesitant about introducing my friends to each other today unless there's a strong focus
to the event.
In lesser amounts, it's just the natural problem of having widely disparate experiences with two different friends - if you're talking about college fun with one friend and talking about past parties with two friend, it's awkward.