If you try to update a NuGet package, and get an error saying:
Update-Package : 'package' was not installed in any project. Update failed.
(but it’s definitely already in use), it may be because of this.
In my case, I’d got into the situation by previously updating the package, and then reverting the changes. Because we’re using the package restore functionality, this meant the package repo contained a newer version of the package than the packages.config referenced. Which didn’t go down well!
The solution is to delete the offending newer package (or just nuke the whole contents of the packages folder (except the repositories.config!), and re-build). You’ll need to close VS first, as it locks the files.
Hopefully a future version of NuGet will provide a more informative message :)
Look on my works, ye Mighty, and despair! 
Log4net?
Que?
I heard there were problems with the versioning on the Log4net nuget package. Apparently they changed the assembly’s strong name without changing the version number or something.
Ah, yes. I think they ended up bumping the version to 2.0. This is purely self-inflicted (sort of).