I definitely recommend the analysis of Texas's derangingly convoluted primary/caucus to which Mike linked earlier, which shows that even if Hillary Clinton wins the state by five points, Obama could still wind up with more overall delegates.
Now, of course, the Clinton campaign has been billing Texas (along with Ohio) as its latest firewall, a crucial state in which victory will give her candidacy a huge boost and will make up for anticipated February losses. But, as any number of people have pointed out, it all comes down to delegates at this point--and it sure looks as though even a big win in Texas would net Clinton only a handful.
Which, it seems to me, leaves open two possibilities:
1) The Clinton campaign has known all along that Texas probably won't help them out much delegate-wise, and their big talk has basically been a ruse to keep donors and superdelegates mollified for a while as they try to figure out some way to change the dynamics of the race.
2) The Clinton campaign simply didn't know how Texas awards its delegates.
I suppose the former seems more likely. But given the degree to which Obama seems to have out-organized Clinton on the ground, and the evident extent to which the Clinton team assumed it would all be over by Super Tuesday, the latter hardly seems inconceivable.