I was once called to the flt deck of a 737 classic for an "autopilot issue" - when i got there both course windows were going nuts and the course pointers on both HSI's were also going nuts. I tried a few resets to no avail. Then I was about to leave the cockpit to consult the books and saw a passenger talking on his mobile in the forward galley (right above the electronic compartment). When we asked him to end his call and turn off his mobile all probs ceased immediately. Never seen anything like it since.