One big source of both delays and bugs is the feeling that your work (which has compatibility impacting changes) _has_ to make it in the current release, because otherwise it will languish for years until the next train leaves.

Likewise, if a fix requires a behavioral change that impacts third party developers, that fix may not feasible to deploy until the next release. Apple had some pretty fundamental things like Cocoa outline views broken for multiple years because the fix required API changes.