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
enumerate) “knows” there are only 6 of these sets.
There are also different label definitions for the
itemize environments at
their own private levels of nesting. Consider this example:
In the example,
enumeratehas labels as for a first-level
enumerate, and is indented as for a first-level list;
itemizehas labels as for a first level
itemize, and is indented as for a second-level list; and
enumeratehas labels as for a second-level
enumerate, and is indented as for a third-level list.
Now, as well as LaTeX knowing that there are 6 sets of
parameters for indentation, it also knows that there are only
4 types of labels each, for the environments
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
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.