Someting exceptional happened.
Something exceptional is not the same as "unusual" or "erratic". Exceptional in terms of a safety culture means that somebody acted with unusual skill or courage to bring an unexpected situation that posed great risk back into the safety zone, or demonstrated great anticipation to avoid such a situation. It is very closely related to "resilience", whereby people are willing to act on an anomaly even before they made a full diagnosis, and demonstrate that they are able to act correctly at great speed and under the guidance of swift negative feedback.
[See "Managing the Unexpected" from Karl Weick and Kathleen Sutcliffe for an excellent explanation of resilience]