Matej wrote:
Could you please some description of the solution here, please, so that it could be found in archives?
Mike noted that, with babel+fancyhdr+natbib the problem occurs in large documents with \renewcommand{\chaptermark}[1]{\markboth{#1}{}} \renewcommand{\sectionmark}[1]{\markright{#1}} \fancyhead[CE]{\MakeUppercase\leftmark} \fancyhead[CO]{\MakeUppercase\rightmark} and seems to be due to the \MakeUppercase command. If you move this to the marking commands like \renewcommand{\chaptermark}[1]{\markboth{\MakeUppercase{#1}}{}} \renewcommand{\sectionmark}[1]{\markright{\MakeUppercase{#1}}} \fancyhead[CE]{\leftmark} \fancyhead[CO]{\rightmark} the problem does not occur. The reason remains opaque. Ekkehart