J Haggerty wrote:

Actually, as a controller, I never considered or questioned an enroute
altitude deviation unless it exceeded 300' or was a threat to another
aircraft. At that point a controller has to determine if the pilots mode
C is incorrect or if he has just deviated from the assigned altitude.

And that is why smart pilots start correcting their altitude just as
soon as the controller asks you to "recycle your transponder!"

Matt