What caused “This save file has been corrupted. The previous save will be loaded”?

Asking because my all shiny save file says it

Pleaselog inorregisterto add a comment.

The fundamental reason save files become “corrupted” is that they fail achecksumat some point. In the context of Pokemon save files, a checksum is a short block of data that is generated using parts of your real save data, after being fed through somealgorithm. (See herefor more detail as it pertains to Gen 4 Pokemon.)

When the game loads your save data, the game re-calculates checksums based upon the save data, and compares them against the checksums it made when it saved the same data. If the checksums don’t match, then the game knows the save data was altered in some way, and says it is “corrupt”.

The game is capable of loading corrupt data, but a failed checksum suggests the data is erroneous. If erroneous data is loaded, unexpected things might happen, so the game is programmed to not load corrupted data.

In Gen 4, the game stores two lots of save data. The first is your “recent” save, and the second is a “backup” save, which is the save prior to the most recent one. The game calculates and compares separate checksums for different parts of the save data structure, from both the recent and backup saves; more detail on that ishere. But to answer this question, I’ll summarise it in this way:

OK, great, butwhywould there be a wrong checksum (i.e. why would the save data have unexpected alterations)? Here are some likely scenarios:

Damage to (or dirt/dust on) the exterior of the cart or the console is more likely to make the console not recognise the cart at all.

Source:As above; also I study computer science, have done ROM hacking, and have used save editors