The 1000 Star Hack (Totally By Me) is a ROM hack created by someone2639.
This hack contains a single level with 1000 Stars in it, which are lined up in rows of 50 (probably), half-clipping into the ground. There are no coins, and the only level geometry is the single grey plane that Mario walks around on. The music used is that of Shifting Sand Land, and the only way to die is to jump off of the edge of the grey plane.
The level played on is called Boob-omb Battlefield, and the Act is called Half Stars Arent Real. The Act name is a reference to a meme circulating within the SM64 ROM hacking community at the time, SM64 but every star is cut in half by Arthurtilly, which was apparently made "to spite Rovert".
Collecting a Star in this hack will play the expected cutscene, then drop Mario back at the starting point for the same level; there is no prompt to save your progress.