Mission Integrity refers to the principle that no action taken by any party to the Mission shall hinder the impression that (at least for the astronauts and Mission Control staff) the mission is taking place in Real Life.
Mission Integrity is paramount, and is best summarized by "The Golden Rule."
The Golden Rule
The Golden Rule states: Nothing goes in or out.
The Golden Rule was established when astronauts would too often find that, due to inadequate planning and/or lack of checklists, they were missing essential equipment, food, toiletries, etc.
Violations of The Golden Rule are a gross breach of Mission Integrity. However, an inventive solution to the astronauts' plight that did not violate The Golden Rule was used during the 2006-07 Mission where shampoo, garbage bags, and several other items were launched in a Rubbermaid capsule, which crash-landed on the Planetary Surface, and had to be retrieved during an EVA. Such solutions are to be applauded, as they resolve problems while keeping Mission Integrity, but they remain imperfect and Spacesim is bound by the Golden Rule to try to avoid such violations.