"Missing number, treated as zero"

In general, this means you've tried to assign something to a count, dimension or skip register that isn't (in TeX's view of things) a number. Usually the problem will become clear using the ordinary techniques of examining errors.

Two LaTeX-specific errors are commonly aired on the newsgroups.

The first arises from a misconfiguration in a system that has been upgraded from LaTeX 2.09: the document uses \usepackage{times}, and the error appears at \begin{document}: the likely cause is lurking files that remain from the LaTeX 2.09 version of the times package. The times package in psnfss is a very simple beast, but the LaTeX 2.09 version is quite complicated, and loads another package: this is clear in the log if you have been "bitten" this way. The resolution is to clear out all the old PostScript-related packages, and then to install psnfss.

The second arises from attempting to use an example describe in The LaTeX Companion, and is exemplified by the following error text:

! Missing number, treated as zero.
<to be read again> 
                   \relax 
l.21 \begin{Ventry}{Return values}
The problem arises because the Companion's examples always assume that the calc package is loaded: this fact is mentioned in the book, but often not noticed. The remedy is to load the calc package in any document using such examples from the Companion.
calc.sty
Distributed as part of macros/latex/required/tools.tar.gz
The psnfss bundle
macros/latex/required/psnfss.tar.gz