Hi Dave,
Thanks for your feedback! I'll dig in a little about what we do.
There's generally two kinds of development. There's new features and there's maintenance. Each carries a small risk of destabilising the software.
We use git for code contributions. When a developer is given the green light to either fix something, or add a new feature they work on it on their localhost installation on a branch of the latest stable release. When they are done, they put in a pull request and assign several reviewers. The reviewer looks over the visual code changes (this catches a lot of logic errors) and also tests the new or changed functionality and reports back any issues in testing (this catches a lot of errors!). When all the reviewers are happy, the change is squashed and merged into the development branch. This development branch is then pulled into everyone's localhosts. This also can find issues not found in the review process.
This means we have three layers of checks.
Now, nothing is perfect and issues do slip by. This may be configuration based where a customer has a specific order of settings we did not test, and this produces a problem. It may also be that while it worked great on our local installs using the latest versions of PHP and MySQL, some older versions throw up issues. There's a lot of moving parts, and a lot of variables when it's released.
After a release, we monitor tickets for trends and investigate them if there are multiple reports on the same thing. Quite often we will spot a problem and patch it within a few hours of it first being reported.
Now that we have an automatic upgrade system which generally means you do not need to log in to our client centre, download files, upload them and so on, we can deliver updates in a very timely manner which are fast to apply. We also have a patching system which shows you any fixes that have been released since you last updated and allows you to apply them.
While there have been some issues that have slipped through our triple check process, I would say that Invision Community 4.5 is very stable for such a big release.
We also put major updates through a beta testing process where we encourage (and support!) early adopters to upgrade to shake out the last few remaining bugs.