Close calls always happen, and mistakes also always happen, as an umpire myself, I would have to respect the calls of my line judges (if any) unless I'm 100% sure their call is wrong, that is the rules of the umpire. If there are no line judges, but the umpire disagrees with the player even though you would give the point to your opponent, just accept that fact and play on, like I said, mistakes happen. If for sure you're 100% sure that the umpire made a mistake, be a fair sport and hit into the net then.