10 CFR 50.65(a)(4)
- (4) Before performing maintenance activities (including but not limited to surveillance, post-maintenance testing, and corrective and preventive maintenance), the licensee shall assess and manage the increase in risk that may result from the proposed maintenance activities. The scope of the assessment may be limited to structures, systems, and components that a risk-informed evaluation process has shown to be significant to public health and safety.
The violation comes from misjudging what the risk of a plant activity was. Typically, something could be discovered to have affected Risk but wasn't accounted for in the model. If this error results in a significant enough difference (detected by a change in risk state) then the discovery could be counted as a violation.