I just bought tickets to Las Vegas on Friday. I got through the checkout process (choosing a flight and a hotel) and then they said there was a "problem" and kicked me back to the beginning of the process. I went through it again, choosing the exact same flights (with the same number of seats available) and the prices were 20-30% higher. The available seats were identical (I was buying for a same-day flight so there were only a handful remaining and it was easy to see that they were the same). I went ahead and bought since it at was the last minute, but I made a mental note not to ever bother coming back.
It's happened so frequently to me on this and other aggregators like Travelocity that it may even be intentional.
Perhaps they display competitive prices so they will be chosen by people who are comparison shopping (either manually or through a site like kayak.com). Then once you've asserted your willingness to buy by moving through the flight selection process they randomly restart you with higher prices on the (probably likely) belief that you have already mentally committed to Orbitz.