I was on a plane with my 2.75 yr old daughter (yeah, the state-machine one). She noticed the air-jet above her passenger seat and switch that controls it.

She turned it on and off.

But to fully test the switch's functionality, she also tried other essential configurations. Such as:

• turning it on and off while standing on one foot.
• Turning it on and off with both left and right hands
• Turning it on and off while forwards and backwards
• Turning it on and off while singing the alphabet song.
• Turning it on and off with one hand while holding a purple monkey in the other hand.
• Turning on other passenger's (mine) switch, while standing from her seat.
• Turning on my switch while standing from my seat.

She has now signed off on the quality bar for the air-jet switches.

Cuteness aside, there's a practical point here: how do you know what configurations needed to be tested? There's endless configurations and unless you have some domain knowledge to properly prune the scenario tree, you  can spend a lot of time testing the wrong things.

Then again, if those jet switches had sensors detecting the presence of purple monkeys, her thorough signoff would have caught it.