I'm not sure if this is an Umbraco core problem or a package problem, but this really shouldn't happen.

I used the packages dashboard in the Backoffice to install a package from the online site (not via local). I knew this package was available both for v7 and v8, but at no time did I get any indication that the v8 package was incorrectly being installed. The installation process hung after a while, and I finally gave up waiting, and hit F5 to refresh. At that point I got the exception messages showing me that there was code looking for v8 classes (composers, etc.).

Fortunately I had a back up and was able to revert the installation this time, but really, come on!

-Tor