When the server steps over the baseline while making a serve.

The server cannot touch or cross the baseline with either foot before the ball is hit. If this occurs, it's called a foot fault. Even if the serve goes in the appropriate service box, once a foot fault is called (usually by an umpire) the serve is dead. If a foot fault is called on a second serve, no matter what, the server loses the point. The server however has the right to ask which foot is causing the fault so they may make an adjustment.