Performing a replace operation with the wrong CAS value returns "Key already exists."

When executing a replace command where the wrong CAS value is supplied the following error is returned,

"Key already exists."

This is a bit hard to figure out the reason for the failure. The Python SDK logs the following for the same error.

"The key already exists in the server. If you have supplied a CAS then the key exists with a CAS value different than specified"