Yoshi

Yoshi is a character that appears on top of the castle in Super Mario 64 after you have collected all 120 stars. After you talk to him, he will give you 100 lives and the special triple jump.

Glitches

Lives Softlock

On the Japanese version of the game only, there is a mistake in the code that will set Mario's lives to -25 if he reaches 1,000 coins. When you talk to Yoshi and he gives you 100 lives, he keeps increasing your life counter until it reaches 100. But if you have 1,000 or more coins while talking to Yoshi, then your lives will continually be reset to -25, which prevents them from ever reaching 100 and thus causing a softlock.

This softlock was first documented by pannenkoek in a video uploaded on July 27th, 2016.

Yoshi Walks off the Roof

Yoshi is only intended to walk on the flat part of the castle roof. However, under some circumstances it is possible for Yoshi to accidentally walk onto the slanted part of the castle, which messes up his movement and causes him to walk off the edge of the castle. After he falls off he despairs, but will respawn if you go back to where he normally is. If you talk to Yoshi while he is starting to fall, then he will fall off the castle without unloading and try to walk to a specific location to jump and finish his cutscene. But he can never reach this location since he has fallen off, resulting in a softlock as Yoshi walks endlessly.

The first mention of Yoshi falling off the castle was in a 2003 SM64 FAQ written by king kid, and the first video of the softlock was a 2007 video by Myles.