When the result was delivered so late that you have been exposed to a large number of possible vectors and infected. Honestly, getting a covid test three days before flying is a practical compromise more than a medically sound procedure. Every compromise allows the virus to spread further and take more lives, so the art is trying to be practical while still doing things that have medical value. If we had been requiring same day tests for the past 6, 9 or 12 months then the market would have had an incentive to deliver tests that were more meaningful. Since we didn't require that, their incentive was to jam as many swabs up as many noses as possible, even if it took a week or more to get results, because they could still charge for every one of them.