In my case, this is apparently caused by a few single quotes in the hnear argument in the hnear url. Removing quotes from the iframe src and a href attributes fixes the problem.
For instance,
<a href="http://maps.google.com/maps/ms?source=embed&hl=fr&geocode=&aq=&ie=UTF8&hq=&hnear=Ch%C3%A2teau-d'%C5%92x,+Riviera-Pays-d'Enhaut,+Vaud,+Switzerland&msa=0&ll=46.472921,7.130213&spn=0.021578,0.055747&msid=208989902893156915814.0004a534bae54848e2335">
has become
<a href="http://maps.google.com/maps/ms?source=embed&hl=fr&geocode=&aq=&ie=UTF8&hq=&hnear=Ch%C3%A2teau-d%C5%92x,+Riviera-Pays-dEnhaut,+Vaud,+Switzerland&msa=0&ll=46.472921,7.130213&spn=0.021578,0.055747&msid=208989902893156915814.0004a534bae54848e2335">
Not sure if Google didn't need an argument in my case, or it could handle the changes gracefully.
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