At this point, the main basis I have for believing the health care reform bill will pass (here's my latest while I was at the Dish...for perspective, here's Jonathan clubber Chait's view, and here's Nate Silver's take) is that I would be shocked if Barack Obama, Rahm Emanuel, Nancy Pelosi, and Henry Waxman moved things to this stage without knowing they could get the votes. We're talking about some very skilled pols, here.
I can't think of any historical precedent for a failure this large in a House vote -- a major policy with the full support of the president and the majority party leadership that got this close without winning. The two most notable floor defeats that I remember are the first vote on TARP in fall 2008, and the vote (if I recall correctly) on the rule on the Bush deficit reduction package in 1990. In both cases, radical Republicans defected from a bipartisan agreement, in both cases between majority Democrats and a Republican president. And in both cases the policy eventually passed.
Another thing that can happen is a bill getting pulled from the floor after a killer amendment is adopted, either because the leadership no longer supports the bill or because the bill no longer has the votes. I can't think of examples from memory, but that does happen.
Neither of those is relevant to this case. What we need are either cases in which a bill gets pulled from the floor at the last minute, or cases in which a bill just goes down to defeat on a floor vote. I tend to think both, especially the latter, are rare, and at any rate I'm not thinking of any cases off the top of my head, but I'm sure there are some. Anyone have examples?