Tonight there's gonna be a jailbreak...on a Super Mario World cartridge
In olden days, cheat codes were something that, for the most part, programmers used to make debugging a game easier (or just to let off a bit of steam) that ended up getting left in the game for the tip mags and lines to devour. The practice has largely died out, but there are still ways of cheating out there, even if they're somewhat more complex...if you can figure out hex code and deduce the in-game value that belongs to an address (or get a program to do it for you) you can cheat anything! That's the more modern way to cheat. However, a YouTuber by the name of SethBling has combined this new way with an old game by jailbreaking a Super Mario World cartridge and injecting a hex editor into it! This has various applications -- everything from manipulating basic values like coins and lives, to fiddling about with all the colours in a level, all the way up to making serious modifications to the levels themselves...that and it's a lot of fun, especially when you do it totally by hand -- as Seth does.
Poor old SMW seems to have quite the rough time of it, alas -- people who create tool-assisted speedruns and coders always seem to have a whale of a time with it and other Nintendo properties. The practice of interfering with the game gained mainstream notoriety at the 2014 Awesome Games Done Quick speedrun event when TAS-speedrunner MasterJun, following a seemingly innocuous set of inputs in Yoshi's Island 2, managed to inject versions of Pong and Snake into the game. There have been further experimentation since, with the aforementioned SethBling injecting a version of the legendary Flappy Bird into SMW last year - notable because the coding was done entirely by hand as opposed to being tool-assisted, which is...yeah, somewhat impressive. Seth definitely puts the million-dollar hands on this game, of that there is no doubt.
Of course, Nintendo games are naturally the ones that often get this treatment -- they're very popular after all, and it's pretty likely that come the heat death of the universe, someone will be playing SMW somewhere. Still, it seems as though this sort of code injection could theoretically be possible for most any game -- that there's always some sort of back door inside where one could program a hex editor and start screwing around with values and colours, or even shove a whole game into another game. One wonders if it would even be easier to do it on some less popular but perhaps more intriguing licensed affairs that likely aren't well coded...why just play around with Mario when you could make a pong game using the 16-bit representations of Wayne and Garth from Wayne's World? Or a primitive version of Kaboom starring Mondu the Fat from Slaughter Sport? The possibilities are truly endless here, and should not be limited purely to the best of the big old N. There are plenty of other games out there that are more in need of the potential improvements.