In our case, if such a thing happens (a few times per year across hundreds of people), a separate branch is created and a team working on that feature is completely autonomous for a while, while there is constant normal work in trunk by everyone else. Team tests their feature and adjacent code to an acceptable beta state but doesn't do any extensive or full coverage because it is impossible. Their code may be reviewed at that point if they request it, but it done as an extra activity, with meetings and stuff. Then they optionally give this build to the general QA to run full suite on it. This may be done in several cycles if fatal issues are found. Then they announce that they will do merge into trunk on days A to B and ask everyone to please hold off on committing into trunk in that time. Around that time they send a mail outlining changes and new functionality and potential or actual unfixed issues. QA teams runs as full cover of tests as possible. Merge may be reverted at this point if it is truly bad. Or if it good, team announces success and proceeds with normal work mode.