English Language · Spotting Errors

Find the part of the sentence that contains a grammatical error: (1) Had the research consortium not been so diligent in their cross-referencing of the longitudinal data / (2) acquired during the clinical trials of the experimental neuro-regenerative serum, / (3) the subtle discrepancies in the patients' cognitive recovery / (4) would have likely been overlooked, resulting in a significantly different conclusion.