Wasnt there a 747-400 that experienced major fluctuation in EICAS fuel quantity indication whilst taxing at KLAX a few years ago, traced to a mobile phone (only after it taxied back on stand and a major delay was caused while leak checks/tank dipping/system diagnosis etc was carried out).
Oh and another thing that is very annoying. When passengers come to check in and expect you to check them in whilst they carry on with their mobile phone conversation. It is downright rude - I have had enough of it. I sit there silent until they finish their conversation now.