A while ago I found a glitch accidentally while playing through DK64. The glitch has to do with clipping past the box that triggers falling in the Mad Jack boss, enabling you to walk on the ground under the pillars that the boss battle takes place on, and take a look at the platforms that take you back up to the level of the fight. I couldn't find the specific glitch anywhere on the internet, so I made a video on it (pardon the questionable iPhone-camera production value):