\marginpar on the right sideIn twoside documents, LaTeX makes sterling attempts to put
\marginpars in the correct margin (the outer or the gutter margin,
according to the user's command). However, a booby-trap arises
because TeX runs its page maker asynchronously. If a
\marginpar is processed while page n is being being built, but
doesn't get used until page n+1, then the \marginpar will turn
up on the wrong side of the page. This is an instance of a general
problem: see
"finding if you're on an odd or an even page".
The solution to the problem is for LaTeX to 'remember' which side
of the page each \marginpar should be on. The
mparhack package does this, using marks stored in the .aux
file.