In non-umpired matches, you are on the honour system. You call the balls on your side of the net. What could be simpler? Alas, quite a lot...
As we well know here at LTL HQ, calling the lines is one of the most contentious issues there is.
Much can go wrong. It cannot be put right afterwards, so if you think your opponent is making mistakes, politely ask if he or she is sure about their calls. And do double check what you are doing yourself. Do you make it clear when it is out or do you keep your opponent guessing? Are you accidentally (or, heaven forbid, even deliberately) winding up your opponent? Are you trying to get your own back for what you see as inaccurate calls?
This funny but acutely accurate video from online tennis training team, Essential Tennis, gets to the heart of what can go wrong. Enjoy!