What if a single transistor fails in a microprocessor?
In commercial consumer grade parts? None. It's tested at the factory, perhaps with a short "burn-in" process to guard against early failures. From then on any permanent failure will kill the device. Fortunately this doesn't usually happen for years if the device is kept within temperature parameters.

For military/aerospace systems, there are various sorts of redundant systems that can detect and recover from failures. Either by a process called "lockstep execution", or by simply having multiple computers like the Space Shuttle's famous five flight computers. (The Apollo project went to the moon with a single non-redundant computer!)

There is also ECC RAM, which is available for server-grade systems and can detect and correct single bit errors.