I would have to find the paper again, but CPU caches can mask the error rate. The cached values also can overwrite any corruption with correct values. This has interesting side effect of protecting commonly accessed data structures and function pointers from causing out right crashing. Same applies to commonly used values in a computation.

Unless you get a bit flip in data structure pointer or function pointer, it just adds an error to computation, but does not just out crash.

Also we are talking only a handful of errors out billions of calculations.

-Edit Also swap space may keep very rarely accessed data from corruption on the other end of spectrum.