Frequently - but not always - on a check for updates, update-notifier reports an error in the notification area instead of showing that updates are available:
"An error occurred, please run Package Manager from the right-click menu or apt-get in a terminal to see what is wrong. The error message was: ' Unknown Error: '<class 'KeyError'>' ("The cache has no package named 'wine1.6-i386'")'. This usually means that your installed packages have unmet dependencies"
wine1.6-i386 is an i386-architecture package on my system, a foreign-arch multiarch dependency of wine1.6. update-notifier appears to not cope correctly with foreign-arch packages.
ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: update-notifier 3.160
ProcVersionSignature: Ubuntu 3.18.0-8.9-generic 3.18.1
Uname: Linux 3.18.0-8-generic x86_64
ApportVersion: 2.15.1-0ubuntu4
Architecture: amd64
CurrentDesktop: Unity
Date: Tue Feb 10 07:22:52 2015
InstallationDate: Installed on 2010-09-24 (1600 days ago)
InstallationMedia: Ubuntu 10.04.1 LTS "Lucid Lynx" - Release amd64 (20100816.1)
SourcePackage: update-notifier
UpgradeStatus: Upgraded to vivid on 2014-12-06 (66 days ago)
Status changed to 'Confirmed' because the bug affects multiple users.