Courts don’t expect perfection from an HR investigation. They just want to see that someone in authority took reasonable steps to learn the truth.
Recent case: Teresa and several fellow supervisors were investigated for alleged insubordination. Teresa was fired. HR had interviewed her and the others about conversations they had among themselves. Teresa denied speaking with the other supervisors, but one had phone records showing that he had called her cellphone on the day in question. Teresa flat out denied getting the call. She was fired because the investigator believed the other supervisor. Teresa sued, alleging sex discrimination.
The court threw out her lawsuit, concluding that the investigator could have honestly believed the other supervisor instead of Teresa. (Taddeo v. L.M. Berry, No. 08-CV-6109, WD NY, 2012)