  Broadbent and G\"oller (FSTTCS 2012) proved the undecidability of
bisimulation equivalence for processes generated by epsilon-free second-order
pushdown automata. We add a few remarks concerning the used proof technique,
called Defender's forcing, and the related undecidability proof for first-order
pushdown automata with epsilon-transitions (Jan\v{c}ar and Srba, JACM 2008).
