When you show the engineer and it works

We have all done it and seen it happen but I don't know its name

Someone has tried and proven that something just doesn't work, it is broken. And so you call the engineer and the first time you try to demonstrate it, it works and then works afterwards every time.

It isn't Murphys Law and it isn't Sods Law but what is it?

I call it Engineer Syndrome but that cant be right