I have had this issue ever since i got the hybrid sienna minivan. When at speed 45-65mph and slowing down (for example approaching an intersection), when the driver presses the brake the vehicle initially slows down using the hybrid regen system and at some point switches to using the mechanical brake. I hold down the brake at the same firm pressure but when the vehicle switches from regen to mechanical brake, the vehicle jerks forward as it starts accelerating briefly before the mechanical brake engages. this causes a panic in the driver because vehicle is not behaving how the driver wants. in my experience i have had to slam the brakes even harder. There is a bulletin by Toyota to fix this but my dealer said that did not apply to my vehicle. I have had issues hydroplaning / skidding when making a turn on wet pavement which i think might be due to the acceleration / jerk forward while braking during a turn. this happened to me again on 9/11, i crashed into a concrete base of a light post in a parking light at 5-10mph. the forward collision warning did not trigger and airbags deployed.