Well if the power goes out, I’m figuring that you don’t have a backup power supply just for your sprinkler system, so the sprinklers should also turn off since there isn’t any power being supplied to the solenoids. Now when the power comes back on, the sprinklers should stay off since the opensprinkler Pi needs a command from the pi board to turn the solenoids on which would allow the water to flow. I’m nearly positive this is how it would work, but don’t take my word for it 😛
First, if power goes out, the sprinkler valves are clearly deactivated, because the valves run on 24VAC from power adapter, they don’t run on batteries.
Next, if RPi freezes, it can indeed leave a valve on. There isn’t a watchdog timer on the OSPi to handle this potential issue. But frankly I don’t know how often RPi freezes. I’ve never encountered this myself.