One thing you’re never supposed to do in tennis is cross over onto your opponent’s side of the court. But that’s exactly what 18-year-old Martina Hingis did during a tense French Open final against veteran Steffi Graf in 1999. Hingis, ranked No. 1 going into the tournament, had taken the first set and was leading Graf 2-0 in the second. She disputed a line call and asked the umpire to check the line. The umpire complied, but Hingis thought she had checked the wrong ball mark and marched over to Graf’s side to point to a ball mark with her racket. As the pro-Graf French crowd jeered and chanted “Steffi!”, Hingis refused to continue playing, calling for the tournament referee. The umpire then issued a penalty point. The incident marked a turning point in the match — Hingis began to crumble, losing the title to Graf. She then retreated to the locker room, bursting into tears into her mother’s arms.