Is it possible for a get to become invalidated? If a formerly married couple were to violate prohibitions of Yichud or Niddah can the get be considered invalid? If so, what is the new marital status of the couple?

A get will be invalidated if: 1. An error was made in the original procedure, for instance the witnesses were invalid or the get was written improperly. 2. The get was given on condition and the condition was not fulfilled (many rabbinic courts will not issue a conditional get to prevent later invalidation. Improper subsequent behaviour of husband and/or wife will not invalidate a get retroactively.