This error appears when you start a LaTeX list.
LaTeX keeps track of the nesting of one list inside another. There
is a set of list formatting parameters built-in for application to
each of the list nesting levels; the parameters determine indentation,
item separation, and so on. The list environment (the
basis for list environments like itemize and
enumerate) "knows" there are only 6 of these sets.
There are also different label definitions for the
enumerate and itemize environments at
their own private levels of nesting. Consider this example:
\begin{enumerate}
\item first item of first enumerate
\begin{itemize}
\item first item of first itemize
\begin{enumerate}
\item first item of second enumerate
...
\end{enumerate}
...
\end{itemize}
...
\end{enumerate}
In the example,
enumerate has labels as for a
first-level enumerate, and is indented as for a
first-level list;
itemize has labels as for a first level
itemize, and is indented as for a second-level list;
and
enumerate has labels as for a
second-level enumerate, and is indented as for a
third-level list.
enumerate
and itemize (this "knowledge" spells out a requirement
for class writers, since the class supplies the sets of parameters).
From the above, we can deduce that there are several ways we can run
out of space: we can have 6 lists (of any sort) nested, and try to
start a new one; we can have 4 enumerate environments
somewhere among the set of nested lists, and try to add another one;
and we can have 4 itemize environments somewhere among
the set of nested lists, and try to add another one.
What can be done about the problem? Not much, short of rewriting LaTeX - you really need to rewrite your document in a slightly less labyrinthine way.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=toodeep