Consider the following MWE:
\documentclass{article}
\usepackage{kantlipsum}
\pagestyle{headings}
\usepackage{longtable}
\begin{document}
\section{First}
\kant[1]
% using this longtable changes the header from "1 First" to "2 Second":
% \begin{longtable}{ll}
% bla & blub \\
% bla & blub
% \end{longtable}
\section{Second}
\kant[2]
\end{document}
Curiously adding the longtable
environment changes the page heading from »1 First« to »2 Second«. Am I missing something obvious? Does anybody know where to look for the cause?
\documentclass{article}\pagestyle{headings}
. I suggest you edit the question/MWE acordingly.