And BTW the newer transponders are creating even more havok than the old.
I have heard quite a few times now aircraft screwing with ATC radar because there Mode S ident is wrong usually with an exam callsign on it. When asked to change it the pilot doesn't have a clue how to do it.
Which for those that don't know is usually mode 2-3 times until the ident comes up then big knob to move the cursor and small to change it.