I help manage several Scientific Linux 6 servers.  They are set to automatically update with PackageKit-cron, which runs as part of its cron job:
   pkcon --background --noninteractive --plain update

Recently, there was an update that was not able to complete.  Instead of exiting with an error, however, pkcon appeared to hang forever.  It may also have been responsible for high load and high IOwait seen on the servers, though I am not sure of that.   I believe it was waiting for user input, even though run with --noninteractive and unconnected to any console.

Running interactively, I got the following results:

$ sudo pkcon update
Updating system               [=========================]        
Waiting for authentication    [=========================]        
Starting                      [=========================]        
Running                       [                         ] (0%) 
Do you want to allow installing of unsigned software? [N/y]    The unsigned software will not be installed.
                              [=========================]        
Fatal error: user declined interaction


or with yum:

$ sudo yum update 
 ....
--> Running transaction check
---> Package kmod-ndiswrapper.i686 0:1.56-1.el6.elrepo will be updated
---> Package kmod-ndiswrapper.i686 0:1.57-1.el6.elrepo will be an update
--> Processing Dependency: ndiswrapper-utils >= 1.57 for package: kmod-ndiswrapper-1.57-1.el6.elrepo.i686
--> Finished Dependency Resolution
Error: Package: kmod-ndiswrapper-1.57-1.el6.elrepo.i686 (sl-livecd-extra)
           Requires: ndiswrapper-utils >= 1.57
 You could try using --skip-broken to work around the problem


The yum error message is certainly more informative, and resulted in a workaround for the problem (removing the kmod-ndiswrapper package).