sudo apt-get update最后给了我这个错误。我正在使用 Y PPA Manager，现在似乎所有 PPA 都可以，但问题（我认为）出在官方 Ubuntu 存储库上：
W: GPG error: http://repository.spotify.com stable InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 13B00F1FD2C19886
W: GPG error: http://archive.ubuntu.com wily InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32
W: GPG error: http://archive.canonical.com wily InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32 …