Traveling can be frustrating to begin with. We've all dealt with searches and delays and cancellations and while we are used to it, it doesn't make it any better.
The last thing I would expect is to book a flight and have it be over capacity but, I guess it happens. I'm not sure how exactly - I mean, wouldn't you only sell the seating available and not additional? Wait, that's common sense and we know how that typically goes, right?