Feather#3
About a year ago I was happily tootling up the Light A/c Lane Nth of BK when SYD RDR gave a call to someone in my position. Intrigued, I answered. Apparently, I was in danger of collecting a QF B767 at 3,000ft on a right base for the 16R ILS. Turns out my Mode C height box was pushing out an ALT readout 1,000ft higher than actual.
This about a year ago is not such a bad thing. Of course as VFR to be in C you needed a clearance so you could be reasonably assumed to be OCTA. Today in E as VFR you do not need a clearance. Sooooo with your faulty mode C reading in todays world the traffic that will get an RA reference you with that faulty readout will be responding to incorrect information and whilst doing everything properly may very well hit you.
Enjoy your tootling.