This counter increments when the switch receives an authentication response whose authenticator or signature doesn't match what it computed, which in TACACS+ almost always means the shared secret key configured on the switch doesn't match the one configured on the server. This is one of the most actionable counters in the whole table for an admin, since a nonzero and climbing value points straight at a misconfigured or recently-rotated shared secret rather than a network or credentials issue. For example, this counter jumping to match every login attempt right after someone updates the TACACS+ key on the server (but forgets the switch) is a textbook signature of this exact fault.