In the interests of research i managed to miss a flight airside. My passport somehow mislaid itself, I told BA, my bag was unloaded, and my flight left for Lagos with an extra empty seat. I went home, expecting the nightmare of reporting the loss and getting a new passport, but the miscreant one was found at Heathrow and BA put me on the next day's flight with no drama at all. I'm sure things are not always handled as seamlessly as that, but it's more anecdotal evidence that the airline can be remarkably forgiving.

But probably best not to count on it.