I’m sure you have seen that one before. In most cases you expect it or at least understand what caused it. In some instances you probably ignore it (we all do, no shame). What if this happens when you log into the CVM or the host? Has cluster security been compromised?
During the upgrade or rescue of the AOS new keys are created for each node in the cluster. When you open SSH session, these keys are compared to those that were noted on the client previously and since there is a mismatch a warning is triggered.
KB-2388 Upgrade/Re-install of AOS changes the ssh key for remote host identification explains how to clean up the keys to get rid of the warnings.