Dragon Ball Z - episode 33 - 23 Jan, 1990

Gohan and Krillin arrive at the battlefield, but have to be quick to save Goku, who is being pounded mercilessly by Ape Vegeta. Krillin devises a plan to stop Vegeta: cut off the Saiyan Prince's tail. As Krillin prepares to fire the Destructo Disk Gohan distracts Vegeta, but the Saiyan Prince is too smart for their tricks, and foils their plot. Suddenly, Yajirobe appears out of nowhere and cuts Vegeta's tail off, causing him to revert to his normal form. Unfortunately, he still proves too powerful to be stopped by the remaining Z Fighters. (Source: Wikipedia)