Comment 6 for bug 1016047

You're right that Wine has had quite the history with regressions, but those are only in the biweekly unstable releases. 1.2 had only one regression compared with 1.0 that I'm aware of, and it was fixed by 1.2.1. We've never had a regression in a subsequent point release.

The reputation mostly comes from the fact that for a long time Wine didn't have stable releases (1.0 was about 15 years in the making), and even then a large chunk of users were on the development releases because a particular app never worked for them in stable.

Anyway, to answer your question, it is a combination of things:
 1) An evergrowing test suite and test-driven development
 2) A large collection of user reports in appdb and bugzilla
 3) A very conservative mindset about which patches can be cherry-picked for the stable release branch: similar, in many ways, to Ubuntu's own philosophy.