One problem related to brake disc caliper has been reported for the 2008 Toyota Prius. The most recently reported issues are listed below.
Driving through the rocky mountains, my brake actuator failed causing the front caliper to seize. I was unable to brake safely. The vehicle swerved during braking and failed to respond in time to prevent collisions. Had I been in a high-traffic area, injury or fatality would have been likely. This is a known issue and Toyota extended the warranty on this part. Unfortunately, though I am still within the mileage limit, the warranty period expired weeks before the problem was diagnosed. I stress that this is a known issue which presents an imminent danger to drivers, passengers, and the public at large.