Glitch worlds
With a little light romhacking you can access 248 glitch worlds.
I found out about the glitch worlds from this video by SMB1 player Kosmic: https://www.youtube.com/watch?v=E6qRbk5Cscw
I used an SMB1 romhack utility to open up access to the glitch worlds:
S.M.B. Remodeler: https://www.romhacking.net/utilities/1319/
In The S.M.B. Remodeler utility, I loaded my dumped SMB1 rom, clicked “General 1,” and under the “Stages” tab, set “Max Stage Select World - World” to 256, checked the “Stage Select Setting - Enable Stage Select at Start” checkbox; then I saved out the rom–a copy, not my original dumped file, of course!
Running that ROM, pressing button A–not the usual button B for the standard World Select you get after beating the game!–cycles the World number at the top of the title screen, and pressing Run will start that world; the numbers go from 1 (or actually, 0, but you have to loop them for that to come up) through 9, then A through Z, then some symbols and mostly tiles from the ROM. ‘p’
(There’s a romhack that starts the game with a world select menu that shows the worlds in regular numbers, but it doesn’t work with my dump of the game; Kosmic demonstrates it in this video https://www.youtube.com/watch?v=lyQjQ8DiBDI and it can be downloaded from author Threecreepio’s github page here: GitHub - threecreepio/smb-glitchedworlds: A world picker for SMB1, hopefully without breaking too many of the levels in the process. )
I ran them in the emulator Mesen in Windows, and was using the infinite lives and infinite time cheats from Mesen’s built-in cheat library. I didn’t use other cheats like walking through walls or whatever, so I didn’t completely explore the few glitch worlds that aren’t fully accessible by normal movement.
The vast majority of the glitch worlds are just standard SMB1 worlds–or sometimes, underwater variants of them–but the pipes restart the stage instead of going to a bonus area. Some of them have missing–or extra–enemies. In most of the stages, if you can get to the end, it will either loop back to the start again, or return to the title screen; some of the glitch worlds do have more than one level, but sometimes these repeat–World “I” in particular seems to keep going and going with repeating levels; I quit at level 7 but in theory it could go to level 256? (World “L” seemed similar, I quit at 3 there.)
The usual warp zones in these glitched versions of the worlds mostly still work–and will warp you to a STANDARD, non-glitch stage, so from that point you’re just playing the regular game, as far as I can tell.
EXCEPT, the right-hand pipe in G-1’s warp zone took me to minus world (“[blank]-1”)!
This RGMechEx video https://www.youtube.com/watch?v=1ysdUajrhL8 summarizes the 6 actually unique, highly dysfunctional glitch worlds, and explains how the glitch worlds are generated.
This Skelux video neatly summarizes the playable glitch worlds: https://www.youtube.com/watch?v=riaf2vwMzMo