I often see output from TeX with the warning
overfull hbox, badness 10000. What does this message mean?
The first answer doesn't really help somebody fix the problem, though. Take a look at:
to learn about
This message means that a line of your document is too long to fit within the horizontal space on the page, and TeX couldn't find a good way to break it apart. This will usually result in text hanging out past the margin, possibly even running off the side of the page. Common causes are long words without proper hyphenation information and long displayed equations.