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This happens when your document is using the standard book or report class (or one similar), and you’ve got a \section before your first \chapter.

What happens is, that the class numbers sections as “<chapter no>.<section no>”, and until the first \chapter has appeared, the chapter number is 0. (If you use\chapter*, which doesn’t number the chapter it produces, the problem still arises.)

If you’re doing this, it’s possible that the article class is for you; try it and see. Otherwise, put a \chapter before your sections, or do away with section numbering by using \section* instead. An alternative way of avoiding numbering is discussed in “unnumbered sections in the table of contents”.