A friend of mine asked me to look at his Planet Waves CT-04 tuner pedal, over a year ago.

It would occasionally go into a mode where there was no sound and no tuner.
A couple of weeks back I dug it out again and played with it at home. I found that pressing the foot-switch slowly could reliably reproduce the problem. So I replaced the switch with one of these:
Apem 8646A

Problem solved.