We transfer ACH files(i.e. paychecks) via SSH(SFTP) to several banks. You better believe I check keys. One of the banks forces key rotation every 2-ish years.

We transfer ACH files(i.e. paychecks) via SSH(SFTP) to several banks. You better believe I check keys. One of the banks forces key rotation every 2-ish years. I absolutely verify it every rotation and delete the old keys. Occasionally it fails, almost always it's something unexpected happening, but occasionally we catch their errors(verified by connecting from various endpoints/DNS queries/etc). We used to call them all the time whenever that happened. Now we just auto-retry on failure in an hour and that fixes the issue all of the time(so far). We only re-try once and then fail with a ticket. Most of us like our paychecks, so we are pretty good about getting that ticket resolved quickly.