Longtable overfilling the bottom margin with longtable and figures

Question

Possible Duplicate:
longtabu and floats: wrong table breaks on pages with floats

In the paper I'm writing I want to format a long sequences of equations with definitions. To have a consistent indentation in the whole paper I have decided to use two-column longtables to be able to break them between pages. However, there are some nondeterministic vertical overfulls on the bottom margins, well represented by this MWE:

\documentclass[12pt,fleqn]{article}
\usepackage[a4paper, left=2.5cm, right=2.5cm, top=2.0cm, bottom=2.5cm, headsep=1.2cm]{geometry}
\usepackage{longtable}
\usepackage[OT4,T1]{fontenc}
\usepackage{lipsum}

\begin{document}

\lipsum[1-4]   

\begin{figure}[htbp]
\centering
\rule{300pt}{200pt}
\caption{Description}
\end{figure}

\begin{figure}[htbp]
\centering
\rule{300pt}{200pt}
\caption{Description}
\end{figure}

\section{Lorem ipsum}

\lipsum[3-4]

\subsection{A subsection}
\begin{longtable}{p{5.5cm} p{10cm}}
  $a^2+b^2=c^2$ & a theorem\\
  $a^2+b^2=c^2$ & a theorem\\
  $a^2+b^2=c^2$ & a theorem\\
  $a^2+b^2=c^2$ & a theorem\\
  $a^2+b^2=c^2$ & a theorem\\
\end{longtable}

\subsection{Another subsection}
\begin{longtable}{p{5.5cm} p{10cm}}
  $a^2+b^2=c^2$ & a theorem\\
  $a^2+b^2=c^2$ & a theorem\\
  $a^2+b^2=c^2$ & a theorem\\
  $a^2+b^2=c^2$ & a theorem\\
  $a^2+b^2=c^2$ & a theorem\\
  $a^2+b^2=c^2$ & a theorem\\
  $a^2+b^2=c^2$ & a theorem\\
  $a^2+b^2=c^2$ & a theorem\\
  $a^2+b^2=c^2$ & a theorem\\
  $a^2+b^2=c^2$ & a theorem\\
\end{longtable}

\end{document}

It's the shortest WE that I can produce.

What could be the cause of this overfull? Why the second longtable just does not break to 3rd page? Is using multiple longtables in sequence error-prone? Can another package provide such functionality?