Super Mario Bros 1. has a lot of glitches. Let's go through a few.
The infamous world -1, AKA world 32-1
WARNING: You can not escape world -1, forcing a reset.
If you are Super Mario, and get hit while dropping the bridge in any castle level, Mario's form will appear weird in the next world. Mario will appear big, but actually be small. Getting a mushroom makes you small, and getting a Fire Flower will make you small Fire Mario. This is the only legitamite way to get a small Fire Mario.