Quite possibly one for FAQ's, but I'll let tightslot do the honours!
I would add I know it's usual (essential?) for an able bodied person to be sat at each exit (CC for main doors, pax for overwings), but are the rules any different for a partially loaded flight? I also know its commonplace for spare CC to take up pax seats at overwing exits if there are no suitable pax to occupy them - my question - have the airline made a booboo worth complaining about?