In tennis, a Double Fault happens when a player makes two serving mistakes in a row.
The server gets two chances to start a point. If both serves are wrong, it's a double fault. The server loses the point when this happens.
Double Faults can occur by hitting the ball into the net or outside the service box. They can also happen due to foot faults.
Some players are known for double faulting under pressure. Anna Kournikova once had 31 double faults in a single match, a record for women's tennis.