From Halopedia, the Halo wiki
Zero-bug release or ZBR is an old school development vernacular when the development of the software transitions from planning and development to release and support phases. In reality, the term is a misnomer as unimportant bugs are still present in the final product. It is the practice of developing software that is maintained in the highest quality state throughout the entire development process.
It is a target set by Bungie to release Halo: Reach to the public without any noticeable major bugs. To test the game, employees are required to play for numerous hours performing tasks any normal player would perform - such as bumping into other AI. They would then re-perform the test in the same conditions another two times to confirm if it is a bug and not a general error.[notes 1]
The ambient AI for the porcupine model was unable to function in the game, so was removed from the game altogether. It had been decided earlier that only content was to be removed to solve bugs unless specifically necessary due to time constraints.
Bug 15163 - off-centre cross hair
This bug was featured in a pre-release screenshot and featured the cross hair for the MA37 ICWS being far to the left. Generating speculation ranging from evidence to a Resident Evil 4: Wii Edition style of aiming with "Project NATAL" (the project title of the then-unreleased "Kinect") to Bungie being lazy programmers, it was revealed to be a bug in the January 29 Bungie Weekly Update. This update also discredited the rumours of Halo: Reach being made for Natal.
Bug 14579 - SPARTAN gender
When a Spartan player under the "Female" gender option would suffer the effects of her shields being down, her AI player model would suddenly be replaced with the "Male" option.
Bug 42748 - Custom vehicle specifications
Like earlier games, Halo: Reach custom games have the option to select specific vehicles to use. When playing on Sword Base with "Mongooses only" being selected, the map's cameras would turn into Mongooses and hang on the wall. This was due to the cameras being in the "vehicles" folder of the game.
Bug 42844 - Kat
Bug 52719 - Credits
After the campaign is cleared, selecting the "credits" option on the menu to replay the credits will cause the "results" menu to appear after skipping the video. This could potentially grant players extra points.
Bug 55893 - Elite model
The player model for Elite players was the location of a bug. ZBR engineers discovered that rotating the player model would not move the legs, which would remain in their original position.