I noticed an error in google webmaster tools regarding a gzipped version of a sitemap. Turns out the
sitemap.xml.gz file is gzipped twice: one upon creation (as it should be) and another time when being served.
.htaccess file has this rule
AddOutputFilterByType DEFLATE application/xml
which I thought wouldn't match a gzipped XML file, but apparently it does as removing this solves the problem.
What am I doing wrong?