General ⇒ decoding an error message

[earl_e]

while compiling a LaTeX file, I get the warning "<FILENAME>.tex:0:Underfull \vbox (badness 10000) has occurred while \output is active []". Line 0 is "\documentclass{sig-alternate}". what does this mean?

[phi]

This means in most cases that there is a page which LaTeX considers too empty. This is mostly the result of some inconvenient page break. It is not harmful by itself, but can lead to ugly page layouts. Try to change something on the page where this occurs, sometimes that can help. Your warning message was post-processed by some program that removed the page number and inserted the (useless) line number "0"—messages like this are not the result of a specific line in the code. Have a look at the log file to figure out the page number (in square brackets) of the offending page and look if you can do something about this page.