The Youtube channel Retro Game Mechanics has done a series of three videos on glitches in Super Mario Bros. One involves using the NES game Tennis, which has a certain property of its code that allows you to load all kinds of funky levels in SMB.
They’re all interesting, but the one that floats my particular boat is the third, which turns into a deep dive in the compressed manner that Super Mario Bros. stores its levels in ROM, and uses to draw them during play in real time.
1. Access Glitch Worlds in Super Mario Bros. via NES Tennis (13 minutes)
2. Exploring Glitch Level Pointers In Super Mario Bros. (15 minutes)
3. Super Mario Bros. Glitch Levels Explained (30 minutes — this is the one embedded above)