It can be a pain in the proverbial and in my experience, has happened quite a few times but as crew you just get on with it and sort it out. I have always, time permitting, gone up to the gate myself and check with the ground staff and they've always been happy to help and the pax have always been appreciative that you've gone out of your way to find them a seat!!!
Whilst we're on the subject of seating, has anyone ever had to deal with moving unsuitable people from o/wing exits?? I once had to ask 2 obese pax to move from the o/wing exits on the same flight - not a nice job
tact and diplomacy came into force THAT day.
Happy flying folks.