Instrument kept turning on
I worked in software for a gas detector company - wear one of these gadgets in a confined space and it would tell you whether you were about to be blown up, suffocated, or poisoned.
In a new model, the hardware team were on a cost cutting binge, and the first thing to go was the on/off switch. After one of the two remaining buttons was held for a couple of seconds, the software took over and tickled a circuit several times a second to keep the instrument alive.
All went well until it was noticed that occasionally the device would come back to life on its own a few seconds after switch off.
The full hardware team worked on this for days, until the extremely sleep deprived HW Manager strode up to me with a fierce grin and a manic look in his eye.
"You're doing it! The power goes off, then there's a kick on the power maintain line!"
I looked at him, wondering when the penny was going to drop, then prompted him. "So the power goes off?"
"Yes!"
"And the CPU clock stops?"
"Yes!"
"Then what could I possibly be doing to toggle that line?"
He didn't say a word, just spun on his heel and went storming back to the workbench.