Man, MK64 was a glitchy mess. One of the very, VERY few high-profile Nintendo releases that I absolutely never want to play again (but would.. if someone REALLY wanted to).
But, to add to the conversation, in 2000 I bought a cheat code book from Blockbuster with all N64 games and their secrets and such, and learned the game had an insane amount of glitch shortcuts. I spent a good majority of the spring messing around with them.
My favorite glitch (and I have pissed some people off with this one) is the DK Jungle glitch, where you veer to the left right before the end of the first lap and use a mushroom to go right into that edge of the exit from the cave. If you do it correctly, you will skip an entire lap and be on the very end of the second lap.
Try it out!