After adding about 30 scanned images into an appendix, I started getting “too many unprocessed floats” errors. Doing a little digging, I found that many of the images were starting to back up on each other. LaTeX was getting plugged up and was barfing.
The solution:
Add this to your top-level file:
\usepackage[section] {placeins}
By using the placeins package with the section option selected, LaTeX is forced to dump all of the unprocessed floats at the end of each section. There are a few other ways to do it with that package but this way made the most sense to me. Doing that, I get no more errors! Well, at least from that problem.