When installing the webmin rpm package the following messages is sent to stderr.
cat: /etc/lsb-release: No such file or directory
it is much likely produced by this line in the postinstall script, because the error output of "cat" is not redirected to /dev/null like in the other cases.
elsif (\`cat /etc/lsb-release | grep DISTRIB_DESCRIPTION\` =~ /^DISTRIB_DESCRIPTION="Linux Mint 6 Felicia"/) {
Thanks for pointing this out - I will fix this in the next release.