A while ago, I was talking with my buddies and the subject was about installing / updating a package using yum.  yum did not find the package that had just been added.  No problem, either yum clean metadata or yum clean all will do.  Err … it didn’t. 

As it turned out, it didn’t work because the repository in question was disabled by default.  The man page for yum clearly states:

Note that "all files" in the commands below means "all files in 
currently enabled repositories".   If you  want  to  also clean  any 
(temporarily)  disabled  repositories you need to use --enablerepo=’*’ 
option.