The GMC state that a significant event (also known as an untoward or critical incident) is any unintended or unexpected event, which could or did lead to harm of one or more patients. 

This includes incidents which did not cause harm but could have done, or where the event should have been prevented, which is significant enough to be investigated by your employing organisation.

It is an expectation that all doctors as part of the requirements for revalidation are required to record and reflect on significant events in their work with the focus on what they have learnt as a result of the event/s. 

You only need to record this information on your form R if any formal investigations have not been completed and resolved, or if they have not been included in your portfolio.