These are some of the common issues that new Arch users experience.
If you encounter errors trying to install an application, such as, "error: failed retrieving file 'foobar.pkg.tar.xz' from extra: Not Found", this invariably means one of two things: either your local database is out-of-date and needs to be resynced with a 'pacman -Syyu', or your mirror is out of date and you need to choose one that is up-to-date.
Arch is a rolling release distribution and you need to ensure that pacman is aware of the latest versions of software. If in doubt, go to the Package Database at http://www.archlinux.org/packages/ and check what the current version of 'foobar' is in the repos.
Pacman gives an error similar to,
error: failed to commit transaction (conflicting files) foobar: /path/to/file exists in filesystem
This is because pacman has detected a file conflict. Pacman will not blindly overwrite files and instead aborts. As a user, you must figure out what is causing the conflict and act accordingly. See
the wiki for possible solutions.
There are two ways to interpret this message: the signature for the package is invalid, so check your keyring is up-to-date and that you are synching with an up-to-date mirror, or the package was only partially downloaded and is indeed corrupt. If the latter, delete it manually from your cache, resynch your local database and reinstall the package with -Syyu $package.
If the command pacman-key --init takes a long time to finish it is because your system does not have enough entropy. The process should normally take no more than a few minutes. See the wiki article on pacman-key. That article also includes information for when operating over ssh.
A guide to downgrading packages can be found on the wiki.
When it is ready.