SLAPAYO is a glitch in Super Smash Bros. Brawl. Naruto[NU] first discovered the glitch on March 30th, 2008. However, on May 4, 2008, the SLAPAHO crew provided the first documented evidence of the creation of the glitch.
The details of the glitch are still uncertain, and there are many questions after reviewing the video. Unfortunately, in their haste to have video evidence of the glitch, SLAPAHO quickly ended the match with little experience of the glitch itself so the replay could be saved.
What is known is that it involves Ness being hit by an attack of higher priority while his yo-yo is outstretched. Shortly before impact, Ness must release the yoyo. The animation will be canceled, causing the yo-yo to fail in retraction, effectively leaving the yo-yo limp at Ness' side. The yo-yo appears to retain its charging hitbox, which only hits an opponent or damageable object once, very similar to the function of the SLAPASTICK.
A similar glitch occurs in Melee with Dr. Mario reflecting a Pokéball to Mario, where Mario's cape "breaks" and his cape hangs out the entire time.