The point is that the booking engine offers you the fare and then fails. If it said "fare not available" then fine, but you're asking for a specific flight, it offers you the fare, you enter all your details, and then it says "not available" and offers a higher fare. It's done it so often to me that I regard it as "bait and switch" rather than the very small chance that someone else booked the last available seat whilst I was entering my credit card details (and any decent database system would have locks that prevent that happening anyway).