There are many reasons why releases fail. Maybe your test pack wasn’t scoped properly or that harmless application patch from the vendor removed a key piece of functionality. All of these pale in terms of avoidability and regularity in comparison to the following conversation:
Requestor: “I want this change to go into the release.”
Release Manager: “Sorry, the scope has closed, we’re scoping the next one though.”
Requestor: “But that’s too long to wait, it has to go in NOW.”
Sound familiar?