it happens more often than you think in places like cuba that people want to escape from. they simply run out to the plane as it's taxiing, or right before take off, and jump in a wheel well right as it's about to take off.
problem is they usually don't know where they're going, so rather than a 3 hour flight to mexico, they get on an 8 hour flight to montreal or something. but i suppose at that point, what difference does it make? it's still fucking cold and no oxygen.