I've had Virgin's online check-in system crash for me before now, during seat allocation. It got around to deallocating my existing seat but not allocating me a new one, then said I'd already checked in when I went back to the beginning and tried again.

I was flying from Hong Kong. The website helpfully gave me a freephone number to call about it… which didn't work from China. When I arrived at the airport that evening I had an… interesting time sorting things out.