I have seen the above "choose a different flight/fare" sometimes with different partners and other times it will go all the way to the booking option and blow up with a "this page has taken flight". In the latter case, I see the booking on hold briefly before it either automatically cancels or a portion of it disappears. I've tried a variety of things like trying different segments by themselves / checking with an agent for availability on those specific flights. In almost all those situations they come back saying there is no availability and/or some of the flights couldn't be confirmed. The non-availability seems to manifest itself in a variety of ways :-)