I used this a lot in early 2000s when my only internet connection was at school or internet cafes, and the data transfer was via a bunch floppy disks I carried

I used this a lot in early 2000s when my only internet connection was at school or internet cafes, and the data transfer was via a bunch floppy disks I carried with me. The floppies often had issues, so a single corrupted byte would normally corrupt the downloaded files (especially images and binaries) but with 10% or 20% of recovery data written in a non-damaged place, it could manage to un-rar. If you're curious how this works: say you have some data like below: 10001100 11001010 11001010 01001010 10001100 11001010 11001010 01001010 You can calculate a sum of each row and each column, mod 2, and store it somewhere. When a single bit gets flipped, with the extra data (assuming the recovery data itself is not borked), you can calculate exactly which bit got flipped. (You can also calculate other kinds of extra data which can deal with more complex bit flips).