It turns out that you can generate enough current with your unpowered stepper motor to spin your 5V cooling fan. :-]
A number of people have warned me not to move the stuff connected to the stepper motors too fast because it could damage the control circuits.
Yeah, not sure this is a great idea…
Yep, over voltage can damage the unprotected 5 V rail components.
You can also cross conntect two stepper. When you move one of them, the other will move too 
If you don’t have any loads or at least a (powered off) PSU connected to your 12V/24V rail while moving the steppers around, that’s a really easy way to fry your electronics.
It also lights up the sensors and display and starts to boot the micro on mine… But yes, bad idea to do it.
A stepper motor is quite a good generator. But the good thing is, efficiency - especially for the smaller motors is limited, so you usually do not generate more voltage than what the motor is designed for. In the end you have a reasonable voltage drop at the resistance of the coils.
I have heard of people making wind turbines with stepper motors before.