Well dj some people do run more than one zone at a time so the hardware should be designed to accept any command sent to it from the software. But you’re right there is a very bad bug in the run once programming. I tried running it (luckily I was standing next to the rPi and was able to unplug it before anything could be damaged), and all the zones started flickering on and off like crazy. It started making the transformer buzz and physically vibrate. I’m using a rainbird brand transformer.
I had my rPi setup to use a 8 channel relay board when it did this so using relays doesn’t fix the problem. And yes, even while using the opensprinkler pi board, running the run once program will cause all of the zones to flicker on and off the same way.