Recently, I saw a video showing a penalty kick replayed twice because the goalkeeper advanced from the goal line.
According to the IFAB Laws of the Game, Law 14 – The Penalty Kick, 1. Procedure:
The defending goalkeeper must remain on the goal line, facing the
kicker, between the goalposts until the ball has been kicked.
These kind of incidences actually happen quite rarely, the famous one being Bill Hamid of D.C. United saving 2 attempts from penalty kick in the game against Portland Timbers but both being overruled by the referee. Here’s the video.