Best practice when testing module updates
A solution that I would very much like to test, but haven't so for, is combining a branching model like this one, not necessarily with git, with selenium automated testing. Every night, a tool like Jenkins pulls the development branch, runs drush make, drush site-install, and runs your selenium test suite.

I also remember reading an empirical study about finding bugs in software, that I unfortunately can not find a link to right now. The findings were, that no single way of finding bugs was "good enough", and that at least two separate models, such as "Test-driven", "User beta testing", "Code review", etc, were necessary to reach a high, +75%, level of found bugs, prior to release.