As pax - when the crew for the flight you are booked on fails to turn up, and you are invited to stand in the aisle of the next one.
As pilot - when, having failed to elicit any response from the tower of what looks like a fairly major airport, you assume a radio failure and commence circling o/h and then notice a motorcycle hurtling up the road from the town nearby. A couple of minutes later your radio miraculously bursts into life...