Use 'supervene' to describe a surprising situation that emerges while something else is already happening. The team had a solid plan, but a power outage supervened.
Pro Tip 2/3
Following Events
When talking about events that follow in a sequence and are not planned, 'supervene' is appropriate. He finished his speech, and questions from the audience supervened.
Pro Tip 3/3
Active Monitoring
Employ 'supervene' when discussing the need to be watchful for new developments in a process or event. While watching the patient, symptoms might supervene.