Many of our non-technical users reported the loss of braking force as 'a surge' or 'acceleration.' All it took was focusing on how to replicate this intermittent problem and then measuring it. Happily, at the same time Toyota released a software patch to the ABS brake system that solved the single-impulse problem.
There have been two reports of "washboard" roads causing a loss of brake effectiveness. But both were from Northern latitudes where frozen ground seems to cause this condition.
In the South, I'm looking for road work where they have cut diagonal strips in the road surface and I can drive on it without road work crews in the area.
I own a Pontiac Vibe (Toyota Matrix) and had the antilock braking activate once when going over a pot hole and starting to apply the breaks. It felt like loss of breaking but I recognized it very quickly. This was on dry road. Not sure if antilock is applicable to the Prius.
An elegant analysis that gets to the bottom of the problem. Potholes certainly present an interesting (and growing) issue because there can be momentary loss of traction due to the lack of substrate as well as the complex interactions with the vehicle suspension. This all results in puzzling traction data being provided to the braking control systems. Your excellent reputation in the hybrid car community is well deserved!