The first and most significant time Rey Mysterio was unmasked occurred after a tag team match against Kevin Nash and Scott Hall for World Championship Wrestling. This unmasking resulted in his performing with no mask from 1999 to 2002.
Rey Mysterio was not happy that WCW made him give up his mask in a relatively insignificant match with little buildup, as Mask vs. Mask matches are among the most significant blow off matches in Lucha Libre. World Wrestling Entertainment insisted that he put his mask back on when he jumped to the company in 2002, even though doing so is a violation of the Lucha Libre tradition. Mysterio received special permission to wear his mask again, and he has since been unmasked three more times, as of 2015.
The first of these three unmaskings was in tribute to his close friend and fellow wrestler Eddie Guerrero, and he looked down so his face was still obscured. The other two instances took place in the ring, and Mysterio simply hid his face from the camera and put the mask back on for his next performance.