I've been involved in both--once.
As a player, the dice were against us all the way. Actually, the chicken thief did escape, the rest of us did not.
As a DM, player stupidity killed the party. No apologies. If they don't know their roles as characters, that's what happens. Short version of what happened? Basically, the party was trapped behind a locked door in a rather large natural room in the Underdark (the Beholder entered from above, around 50 feet up), the thief decided he was a fighter and attacked the Beholder rather than picking the lock. Well, thief went down, no one left to pick the lock, and all were trapped in the room and killed. Again, very short version of a really detailed encounter.