You have a Debian 8 installation and get an error like the following when you want to install or update the owncloud client?
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://download.opensuse.org Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 4ABE1AC7557BEFF9 W: Failed to fetch http://download.opensuse.org/repositories/isv:/ownCloud:/desktop/Debian_8.0/Release W: Some index files failed to download. They have been ignored, or old ones used instead.
Execute the following command and try it again.
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 4ABE1AC7557BEFF9