In the "Arrival of the Saiyan Warriors" section of the "Return of the Saiyans" saga, you end up fighting Nappa a total of three times;

• The first time, you fight him (100% HP) alongside Piccolo, and have to knock his health down to 50%. • The second time, you fight him (100% HP) alongside Krillin and Kid Gohan, and have to knock his health down to 30%. • The third time, you go up against both Nappa and Vegeta at the same time, alongside Goku.

During this third fight, Nappa begins with 30% of his max health, i.e. the amount he was left with from the previous battle. Once you hit him, he instantly goes into "Dark Aura" mode and his health rapidly regenerates back up to 100%. As you cannot hurt him at all while he's healing, it suggests that he is meant to have all of his health before you fight.

HOWEVER, if you manage to do a SINGLE ATTACK that deals 30% or more damage, it will instantly knock him out before he even gets the chance to heal. Having tested this with both ki-based and physical fighters, it seems that it's harder to pull off as a ki-based fighter, even at level 80 with 100 in ki blast stats and Z-soul/clothes that boost it further. On the other hand, physical fighters, particularly those at level 80 with 100 in basic attack and Z-soul/clothes that boost it further, are able to pull off the 30% damage in a single basic punch.

This 'glitch' can reduce the time of the Saga by a decent amount. Nothing too severe, but it makes a difference. After all, you'll not only save the time of having to beat him from 100% hp to 0%, but also the time it takes for him to heal up.

NOTE: This glitch works as of the 18th of March 2015.