Failure behaviour in systems falls into one of the following categories:
Shut down and prevent further operation, common in security-critical components as it ensures no transactions are processed without correct vetting.
Continue operations as if the failing system were not in place. This approach is common where availability is deemed more important than security.
In a failure condition the system responds in a way that causes minimal or no harm to other equipment, the environment or people. Where this isn't possible, redundancy will be employed, allowing operations to continue as if the faulting system was not in place.
Recovery by failing over onto another component.