It's never happened to me when booking on UA or AA's websites, however that could be entirely coincidental.

My complaint wasn't so much about losing the initial price, as the way it described the failure to book as something different than what it was. And also the fact that the site had no easy navigation to allow me to fix the issue, and instead I had to start from scratch including re-typing in all of my passenger contact and billing information. Computer system UI's are a big soapbox issue for me, as is customer service in general.