When a team is working to hit ZBB at the end of a milestone, that team has at various points gone through a number of bug bar level changes limiting the quantity of bugs to those bugs directly impacting the milestone's most critical release criteria. During the timeframe right before ZBB is hit, there is a lockdown period where developers must get permission to check in their bug fixes.

During this period of lockdown, each new bug coming in gets questioned to its importance for the milestone's release criteria. If the bug does not meet the milestone's release criteria, that bug is put into the next milestone. This is where it is very important to have the proper release (or exit) criteria defined for the milestone prior to reaching the end of the milestone.

Without having the proper criteria defined ahead of time for each milestone, it is impossible to properly hit ZBB due to the constant incoming flow of bugs albeit at a slower rate as the bits stabilize. So, plan your exit criteria ahead of your milestones and match the release criteria to the features being coded during the milestone and don't forget about your previous milestones either to have a better path to hitting ZBB as scheduled.