Originally Posted by
ve3id
Mode C data are not corrected by the altimeter setting in the cockpit, but adjusted by computer at the ATC facility based on current altimeter setting - therefore the reading that was shown on their screens would be the actual altitude - ergo no excuse for the ATCO to not notice.
For avoidance of doubt, the adjustment that's made to what the ATCO sees on their screen is based on the prevailing QNH, not on the altimeter setting in the cockpit (although the baro setting is also typically transmitted to ATC via Mode S/EHS.