One problem related to brake disc pads has been reported for the 2011 Toyota Prius. The most recently reported issues are listed below.
While driving on the highway, without warning, I lost power assisted breaking ability - accompanied by a number of dashboard warning lights. I was able to safely stop the vehicle using manual breaking. I had the code checked and have learned that the (code 1391) brake accumulator failure is a common issue in the Prius and has been subject to a recall/replacement for the years prior to my make (I believe 2008-2010). Clearly this is still an issue and the sudden/no-warning failure of the system causes an alarming and potentially dangerous loss of breaking control during high speed driving. Luckily, I was not using the breaks at the time of the failure and was able to determine that the manual break pads were serviceable but I needed to rapidly increase the pressure on my breaks to slow down sufficiently to avoid an accident.