(preface needed! )
You have to specify \maketitle after \begin{document}.
This can be caused by forgetting a closing brace.
This can be caused by forgetting the \ on \begin, even though the document has the right number of { and }.
In LaTeX, \: is a math mode operator which inserts a ``medium'' space. So constructs like this:
\LaTeX\: % missing $ errorresult in the mysterious Missing $ inserted error. It's advisable to get in the habit of using {} after abbreviations to minimize problems like this, as in:
\LaTeX{}: % ok
If [...] follows \\, even with intervening whitespace and newlines, it will be taken as an optional argument, and thus expected to start with a number. (Even if amsmath is loaded, apparently, in contrast to the The LaTeX Companion.) For example, a math alignment like this causes the error:
a^2 & a^3 \\ [b^2] & ...One remedy is to insert {} before the left bracket.
Using tabular, this can be caused by an \hline without a preceding \\. Similarly, using booktabs, this is the result of leaving out the \\ in the line before \bottomrule.
Perhaps you left off the braces after \section. It should be used like this:
\section{The section title}
This happens when hyperref is used under pdftex and a citation splits across a page boundary. To fix it, note the page number of the error and specify the draft option to hyperref so you get an output PDF. Then you can see the citation in question and rewrite to fix it. (Thanks to James Bednar's posting on the pdftex list about this.)