Released: 1990, SNES
Super Mario World was Nintendo's first title for the SNES, and it was rushed to release for this reason, leaving a lot of unfinished content.
It contains a wealth of unused levels, some of which are from extremely early in development. There's also a lot of unused objects, from a classic Piranha Plant to a ceiling full of Swoopers – all the way to an unused screen-sized cage!
All this, combined with a huge amount of interesting early assets that leaked in 2020, and stuff from other sources like magazine preview screenshots, reveals that Super Mario World was originally intended to be a very different game: Super Mario Bros. 4.