My teammates (USTA league match) had this issue last night, I wanted to see how you would handle it. I know what I would have done. Player A & B vs. C & D. A hits ball deep to C near baseline. C returns ball over net, but B intercepts and hits a winner. D, who was at the net, then goes to check the mark from A's shot and declares that is was out and takes the point. We all know what the rules/code/common sense say, but what would you do if you were in that situation? There are no linesmen/umpires/officials around. It's you vs. them. For me, my exact actions would depend on some things. Is the match close? Does this point lose/win a game for me/them? Have there been other "bad" calls? If it was a close match and/or a breakpoint either way, then I would stand up and say no way. I always carry a rule book (Friend at Court) and have not problems bringing it out. If the call meant nothing, 40-0 in a 5-0 set, then I would at least let them know that the call was improper, but allow it. Sometimes better just to get a match over with, than to start an argument. How far would you be willing to take it?