[pauldino]

That's not the case for JPL missions (the paper originally is from JPL) which generally have 2 separate independent computers where 1 is active at a time.

Since they're independent, the 2 computers don't actually have to run the same software, I believe during Mars entry descent and landing the standby compute element runs a different less sophisticated but easier to validate version of the EDL code to take over if any fault is detected while the primary software is running. (I was going to do a quick check on dataverse.jpl.nasa.gov to confirm that but it seems to be down)

Also I think a few years ago on the Mars Curiosity rover (2012) there was some corruption in the flash storage on one of the computers that prevents the full flight software from being loaded on to it, so instead it runs a stripped-down version of the code with very limited functionality to function as a lifeboat in case the fully-working computer ever fails. https://ieeexplore.ieee.org/document/9843266