You'll recall in this post, I described how the high performance Pulsestar spark plugs from Enerpulse blew out the ignition coil on my 2002 Prius. I sent in the coil and the plugs to Enerpulse and requested they make good on their guarantee that their plugs would not harm your engine by reimbursing me for the cost of the plugs and the repair. To my surprise, I received a very nice letter from Enerpulse apologizing for the damage and a check reimbursing me for the cost of the repairs and the plugs.
It seems that some instances of their older plugs occasionally had a problem with resistance growing over time, and if the resistance becomes high enough, it causes the plug to wear down fast and eventually misfire, which can then cause damage to the coil. This explains the rapid deterioration in mileage and performance I was seeing over the summer, followed by the catastrophic failure.
They also offered a 10% discount on their new G3 version of the Pulstar plugs. These have improved lifetime and better performance. I am tempted, since the torque and pickup on the 2002 Prius is terrible, but after one bad experience with the product I am not sure whether it makes sense to try it again. I still have a set of their original plugs in my 2008 Prius, so far, they seem to be performing fine.