My worst test problem was a TV text display circuit in the 70s. After a lot of tracking down I knew that there could be no pulse coming from a monostable but when I looked at my scope there was the pulse.
This had me puzzled for quite some time until I discovers that the action of putting my scope lead on the monostable pin caused it to work and that the display had actually sprung to life, but I wasn't seeing it as I was looking at the scope.
Monostable chip replaced, problem solved, but it had me going for some time.