Houston had the ball and was down 101-100 with 13 seconds left in overtime when reserve player Winston Garland walked onto the court during the play and became his team's sixth man.
Video from the game shows that Garland jumped off the bench, went onto the court, and tried to illegally play during the final seconds of an NBA playoff game.
No one caught it at the time, but then-Sonics coach George Karl said later he was dumbfounded when he was watching the film a few days later and noticed it. Garland himself still refuses to acknowledge that any cheating took place, despite this video.
The play ultimately came to nothing, and the Sonics won the game and the series. The refs missed the extra player and so did most everyone else for the last 19 years.
Here's the video. The play starts at about the 16:48 mark. As you watch, keep an eye on the Rockets bench. Garland is kneeling on the sidelines before he gets up, takes the court, and gets in a rebounding position when the shot goes up: