It's always been a problem in any game people play.
At the Bowling Alley when my buddy starts to lose he is all, "well, you known my arm done been hurtin' me." Or out shooting skeet, "this here gun sight seems to be off a bit because I done cut that sucker dead center!" We all do a little something like that when the game stops being competitive or fun. I was out racing my nephews a few months back (half my age) and I've always left them far in the dust. But now they are older they can run like the wind also. Darned if I didn't just happen to drop my keys when we were racing last time! I had to stop and get them which caused me to lose
I assured them had that not happened I'd have overtaken them both. Once I know a game is "decided" I usually offer my opponent a way out (resign) to save us both time. When they decline and then the game drags on with slower return times is when it starts being a problem. I will always just keep returning the moves until the opponent either concedes or vanishes completely for a few weeks. Then I just consider that a concession and move on to a new opponent.