This whole reason is why Intamin has gone back to permanent earth magnets on their coasters. Take the roll back brakes on Dragster. They take air to lower to allow the train to launch. After the train passes, the air evacuates and springs or something of the like shoots the fins up. In the event of a power failure, and air pressure loss, the brake fins will shoot upwards on their own.
Now I hear even this sytem can fail - but I don't know if it is true. Rumor is - this last really bad breakdown Kingda Ka had - it launched and something happened during the launch. The brake fins did not raise, but thankfully the train made it over. Like I said I don't know if this is true, and I like to err on the side of skepticism - but it is just what I heard.
And even earth magnets can fail. There are a few natural phenomena that can cause this - such as the north and south magnetic poles flipping.
Edit: Dur! I didn't see page 3!