If wrong credentials are provided during discovery, the authentication for access to the device will fail. OpManager will not be able to fetch the device type and hence the device will be classified as 'Unknown'.
If the credentials are incorrect / has expired after discovering the device, OpManager will not be able to access the device and hence data cannot be retrieved for monitoring. This restricts OpManager from monitoring the performance of the device to which the credential is associated. However, the availablity monitoring of the device will continue.
In case of WMI, if the network has a incorrect credential threshold limit (the maximum number of incorrect attempts allowed), the account will be locked in the Active Directory when the threshold limit is reached
Also, incorrect credentials will impact the performace of availablity monitoring.
To avoid this, you can periodically run Test Credentials to make sure the credentials are up to date. To schedule a new Test Credential routine, go to Settings > Discovery > Test Credentials.