I was having real problems trying to cron schedule an rsync backup.

After much investigation and thanks to zcat and BobJensen on irc.freenode.net#fedora, I managed to get it working.

My rsync command called a remote machine via ssh using a shortcut specified in my ~/.ssh/config file, so the command was:
rsync /path/to/source shortcut:/path/to/destination

When I ran the command from the terminal it worked fine, but the cron job failed with the errors:
Permission denied, please try again.
Permission denied, please try again.
Permission denied (publickey,gssapi-with-mic,password).

The problem was, I hadn't specified a key in my ~/.ssh/config file, so when I ran the command from a terminal my ssh-agent was providing a key, but that was being picked up by cron. So I added the IdentityFile to ~/.ssh/config and it worked like a charm (it's a passwordless key obvoiusly).

Hopefully this might help somebody else with the same problem.

Cheers - Callum.