The same thing happens to me when I use the run once program. The valves were going on and off like crazy and then the transformer just started vibrating and buzzing very loudly. Luckily I was standing right next to it so I was able to unplug it before it could damage anything like the rPI, the valves, transformer, or even cause a fire.
Maybe the run once feature should be taken out of the program while it is being fixed.