I know there were a lot of questions regarding underfull hbox, but in my case error
Underfull \hbox (badness 10000) detected at line 5
[]$[] $
point to line
5: \begin{document}
How can hbox be underfull if compilation of the document has not even started?
EDIT: after inspection I have found out that autonum
package is creating this error.
\documentclass{article}
\usepackage{autonum}
\begin{document}
\end{document}