Sorry about this for the regular flyers - but I am posting it so save peoples skin, airframe or hopefully just embarrasment (for you) and/or paperwork (for me).
I am not sure what training is given to newer pilots about operation of Transponders, but this is a problem I am seeing on about a dozen occassions every shift - often close to vertical or lateral CTA boundaries. And it is getting worse for some reason.
Please ensure the Transponder is selected to
ALT when you line up - not just ON.
All those aircraft with TCAS get very twitchy if you are near the approach path (and the controllers) and you are not showing
ALTITUDE.
(In G airspace in non-radar areas, or even in E airspace it might be your last line of passive defence)
*The only time your transponder should be switched only to ON in flight is if you are instructed to Squawk Alpha only when previously advised that the Altitude reporting is out of toelrance by the Radar Controller (tolerance +/- 200FT Displayed [Qnh Adjusted])
Transponder Basics (from Avweb)