If for a given repo you set "enabled=0" -- e.g. to force the use of
FTP cf. Bug #138535 -- and that repo has a "mirrorlist" defined, yum
will attempt to fetch the given mirrorlist if no enabled repo has one
defined (i.e. it has only "baseurl" defined).

e.g. on a default yum-2.1.11-4 install in Rawhide, if you...

1. delete the "fedora-updates" and "fedora-updates-testing" repos,
2. set the "fedora" repo to "enabled=0",
3. for "fedora-devel", remove the "mirrorlist" and set an (FTP)
   "baseurl" instead

...yum will try to fetch the "mirrorlist" URL given for "fedora"
despite the repo being disabled. In current versions this means it
will produce a traceback due to Bug #138535, but even with the
Python issue fixed this behaviour is undesireable.