When should you tell an open mic you can’t make your spot?

Curious what people think is reasonable here. Stuff happens and nobody expects a new act to have a tour manager, but a late cancellation can leave a running order with a hole in it. If you realise you can’t make an open mic, is the rule basically tell the organiser the second you know? And if it’s genuinely last-minute, is a short message enough or should you explain what happened?