I turned on my gigantic magma pump tower for the first time and filled my magma plumbing.
Then I turned that off, because it maxes out my power system when it's running.
Then I turned on the power to fill my brand new magma-based goblin execution room.
It didn't fill all the way, because I drained a lot of magma out of my plumbing system, and the magma was flowing as slowly as magma does from the extreme ends of said plumbing towards the intake for the execution room.
So I ordered the main pump tower switched on, and the execution system power switched off.
I expected that if the main pump power lever was pulled first, the power system would be overloaded, the execution room pumps would stop, and the main pump tower would not activate until the execution power was turned off, bringing the power system's load below its maximum.
Instead, after the pump power lever was pulled, my millstone became unpowered and announced that it had removed its jobs, but all pumps connected to the now-overloaded power system briefly ran, including the newly-activated main magma pumps.
Result: Plumbing is immediately pressurized with 7s of magma. Execution pump continues operating, now with a pressurized intake that keeps it from having to wait for more magma to flow under it. The execution pit is quickly pressurized with magma which then overflows into the anteroom where caged goblins are waiting to be thrown into the magma. After a few moments and a shitload of magma, everything on the power system actually stops.
No dwarves were in the room when this happened, the entire area is behind nickel doors in case of just such an unforeseen emergency, the system never leaked magma into my power distribution system (as designed), it was actually pretty great when smoke started coming out of my iron cages, followed by XX{goblin bones[6]}XX shooting across the room following the magma flow before disappearing, and I learned a valuable lesson about performing operations with my magma system one step at a time.
But pumps probably shouldn't do that.
Edit:
Just to be clear about how I know that it really happened just the way I describe, I paused in the middle of the magma flood, looked at all of the pumps involved as well as distant parts of the power network and saw inactive, 1600 total power and 1647 power needed, then one-stepped a few times and observed that magma was, indeed, being sucked out of the bottom of the magma vent and continuing to flood the area I didn't intend to put so much magma in, despite the pumps reading inactive.