### Missing `\``begin``{document}`

Give it a file of plain text, or a LaTeX file that really does have no `\``begin{document}` command in it, and LaTeX will produce this error, quite correctly. LaTeX needs `\``begin{document}` so as to know when to execute the commands that finish off the document preamble.

Other than that, the error can occur as a result of an error of yours, of a corrupt `.aux` file, or of a buggy class or package.

The errors you might commit are absent-mindedly typing a document command (such as `\``section`) in the preamble of your document, missing the comment marker from the beginning of a line, or giving too many arguments to one of the setup commands related to the class or a package that you have loaded.

A corrupt `.aux` file might be due to any number of things; delete the file and to run LaTeX again, twice. If the error recurs, it's probably due to a buggy class or package.

If the error occurs while LaTeX is reading a class or package, then there's probably a bug in the file. The author of the class or package stands the best chance of finding the bug, but with luck you (or someone you ask on a mailing list or on comp.text.tex) will be able to spot the problem and provide a work-around. Always report such bugs, even if you have found a work-around.

This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=missbegdoc