There is a 10 mile stretch of the 91 fwy, the last 10 miles before you approach the 55 interchange, where instead of carpool lane they have 2 toll lanes. These toll lanes require a transponder, you cannot pay cash to use them. If you have a FastTrak transponder that works elsewhere, such as San Fran or New Jersey, I have heard that it will work on the 91 also, but you'll want to verify that first if you have one.
There will be very clear signs before you get to the toll section so you know to get out of the carpool lane, as long as you pay attention - the beginning is just past Corona. The lanes are on both east and west 91, so going home you'll have the same thing.