Super Mario Boards

Being able to kill lakitu twice. I don't even think it's documented on the wiki yet. There is this thing called a lakitu retreat zone. When you cross it, lakitu retreats. Even if you've already killed him. In 8-2, the zone is conveniently next to a spring and a bunch of high blocks so while you are fire Mario, you can hop up there to stomp on lakitu, then when you cross the lakitu retreat zone and he will pop right back up to retreat. Then you can kill him again with a fire ball.

Some more of my favorites:
Minus world
Jumping in between a flagpole or vine
Small fire Mario

What's your favorite glitch?

The 256 extra levels glitch. I think how you do this one is you have to take out Super Mario Bros. in the middle of a playing session, put in Tennis, play a match, take it out, put Super Mario Bros. back in, and then I think you can play the 256 extra levels. These weren't even intended, so they're really glitchy. Also, I think the reason why it's specifically 256 levels is because of byte sizes and things. My other favorites are the Minus world glitch (Especially the Japanese version), fire mario, and the double jump.

Oh yeah forgot about the double jump. That's probably the most practical one that I regularly do.

I feel like there isn't a huge reason to do it while playing, but it's still fun to do.