Special return values for mutex-handling functions

There are two other special-purpose return values which are not returned by any actual Cryptoki functions. These values may be returned by application-supplied mutex-handling functions, and they may safely be ignored by application developers who are not using their own threading model. They are:

CKR_MUTEX_BAD: This error code can be returned by mutex-handling functions who are passed a bad mutex object as an argument. Unfortunately, it is possible for such a function not to recognize a bad mutex object. There is therefore no guarantee that such a function will successfully detect bad mutex objects and return this value.
CKR_MUTEX_NOT_LOCKED: This error code can be returned by mutex-unlocking functions. It indicates that the mutex supplied to the mutex-unlocking function was not locked.

RSA Security Inc. Public-Key Cryptography Standards - PKCS#11 - v210