If anything can go wrong, it will." Murphy's Law (or Sod's Law, as it is known in the UK) applies to big data projects, as well. When those projects concern someone's health, something going wrong in the data can lead to something going very wrong with the patient.
The more one relies on the accuracy of the system, the higher the potential for error. Electronic health records (EHRs) are considered a boon to data aggregation, but they hold a potential downside. Read more here