Here's something I suffer from regularly:
Enter a T2 mission lobby, and find two other players waiting there. We chat for a while and then go and stand on the party pad. Under the "Progress down to the next level" screen, I notice once I've clicked the big blue button:
"Good luck, you'll need it"
"Bye"
They return to haven, leaving me to try to solo the roarmulus twins. This happens once every few days, and I don't even play that much.
I'll be suprised if this hasn't been mentioned on here before, but I haven't seen it. Anyway, This entire problem could be voided by removing the "return to Haven" button when starting a mission from the lobby.
To be honest, I've had to bail on parties before due to IRL problems. The only other option is just force-quiting the game instead, if you remove the "return to haven" button. It's more a convenience thing.
People who are enthralled in efficiency skip the boss because they don't get enough crowns out of it. I personally disagree, because Tokens are worth something, but that's what people do.