I've been working with some tutorials in a book that I've purchased (Poser Pro Pack f/x and Design). After playing around with both the library preset poses for Posette and the generic female clothing in the library, I've noticed that some clothing has a tendency to warp quite badly or fall off entirely with certain poses. This is even after I've opted to conform the new clothing figure (titled Sweater or whatever) to the main model. Is this an error of mine or does it have something to do with the clothing or the model? I haven't worked with designing anything of my own yet and am looking forward to being able to use what I make for a variety of poses.

Yes this is normal for some clothing items. The more extreme the pose the more likely it is to happen.

On the other hand, make sure you have the figure selected and not the clothing when you apply a pose. That can cause a mess as well.