I was at a WTT tournament recently playing a guy when the official called a foot fault on his 2nd serve. The official intended it as a warning thinking it was a 1st serve, but it cost the guy a point instead. Later in the match he called a couple more on the guy too. Another time at USTA states, a couple of us asked a nearby official to monitor a player who was obviously footfaulting. The official went on to call a couple of foot faults on the adjacent court but never on the court we asked him to monitor.