This has happened to someone at work and whilst I was, of course aware of the need to isolate with symptoms, until a clear test result is received, I haven't spotted this cycle of events:
Day 1 mum gets a cough and spends a 24 hours trying to book a test. She and the rest of the household isolate.
Day 4 clear test result received and everyone goes back to work and school.
Day 8 daughter develops a cough, likely caught from mother who tested negative, but still needs to isolate, along with the rest of the family and get tested.
Three more members of the family for it to work through yet...