Personally, I always give the person the benefit of the doubt. However, I've played on clay and had people pause (for example on a serve or a "winner"), walk up to the mark, and then call it out because they could clearly see the mark. I never thought anything of it until I was hitting with the USTA coordinator earlier. He said that calls have to be instantaneous and you can't use a mark as a delayed call. He said the same thing happened to him at nationals except he was already sitting on the bench for changeover when his opponent finally decided the mark was out. He looked to the umpire and the ump said the call had to be instantaneous as well. Is this true? I always thought one of the "benefits" of clay was to go back and check marks..