Interference from Mobiles
Can say with pretty near certainty that my mobile (in my shirt pocket) has interfered with the transponder signal. On one occasion outbound from Luton VFR in a PA-28, the mobile phone sound in the headphones corresponded with ATC saying our transponder signal had just disappeared. Turned phone off, somewhat red faced, and ATC confirmed transponder was back on. This in a well equipped FM immune aeroplane.
Don't know what else it could be as the transponder never failed before or since.
In an earlier life of mine, mobile phones reqularly played absolute havoc with water treatment works instrumentation when within 6-8 feet. The chlorine analysers instead of giving a steady 0.25 ppm readout, suddenly started giving all sorts of wierd results in the 10 - 50 ppm range. This was on triple validated equipment that was supposedly fully RF protected to some BS or other.
The effect was only noticeable over short ranges though, and on an aeroplane I would doubt that anybody more than 10 feet from the equipment racking would have any effect.