Assume I have no mode C. Mil controller "clears" me through the MATZ @ 2500' QFE. I say thanks a lot, but as above, I decide to stay on the QNH which is 12 hPa higher than the QFE value and don't tell the controller. The airfield that I am flying upwind of has a four jet ready for departure and the approach controller, aware of my presence, has applied a climb-out restriction of 2000' QFE to that aircraft. Because I have been stubborn and "don't like flying on QFE", the separation between me and the four jet is now greatly decreased and that pilot, expecting 500' clearance below me, now has the joy of a close encounter of the airprox kind.