A problem I face on a regular basis is trying to test sites that have recently had a lot of module updates. My usual process is to take each module on a case by case basis and test site features related to that module.
For instance, if the wysiwyg module is updated, I'll find a node that implements it in a field and do exploratory testing. But the reality is that the issues can be more subtle - perhaps certain user roles have lost permissions, allowing them to embed media when they were previously restricted.
Sometimes there are so many factors to bear in mind it is impossible to test them all, and invariably the client, at UAT phase, discovers major issues that I may have been blind to for various reasons.
I would be interested to hear of any advice or approaches towards testing sites that have had comprehensive module updates - what can I do to ensure quality, and even, what kind of line should I take with our clients to prepare them for issues that do invariably arise?