Had an issue recently where a server had failed and the ping sensor was obviously Down, however, even though the server was off the entire time PRTG would periodically switch state to Unknown and then back to Down.
The problem being of course that rather than remaining Down until such time as it was confirmed to back up again, the flicking between the two non-Up states caused new notifications to be sent every time it switched back to Down again. I don't understand how or why the sensor would change state when nothing had changed with the server state... it was shutdown the entire time.
I guess I can understand the use of going from Up to Unknown, where PRTG doesn't yet know if something is definitely down, but I don't understand the point of the reverse.
The Unknown notifications all showed "No data since <2 minutes previously>", though I'm not sure what data it thought it had received two minutes ago since the server was off, and there's nothing showing in the logs to indicate it receiving any response.