RAM and ROM can be nominally protected with ECC, which many rad hard processors provide. It's the internal state of the processor (registers and control logic) which is not fully protected. The inherent hardening of the process makes faults less likely, but they are still guaranteed to happen. There are design approaches to improve reliability with triplicate flip-flops but you pay a power and area penalty. Higher level redundancies aren't always practical also due to mass and volume constraints. At the end of the day software has to account for running on an unreliable system and minimizing bad stuff happening as much as possible.