Most of these problems usually grow from a small matter, then escalate. Get your facts straight and be confident in front of the pax, that usually helps. I've lost count of the number of time a passenger will tell you 'THIS ALWAYS HAPPENS WITH THIS AIRLINE' - means, I've had a long drive to the airport, check in wouldn't let me take my bags into the cabin, we're going to be five minutes late, my boss is on my back for something, it's raining, I've never flown with this airline before but your going to catch it anyway!
Note: don't give them the cabin crew winning smile it P's them off even more.
Take comfort in the fact that most of the time you only need to know these people for a few hours, the person next to them may have a whole lifetime with them.