I was meant to fly PHX-JFK-LHR-SOF on the return part of a journey. AA cancelled the PHX-JFK sector while I was in their lounge. Rebooked PHX-LHR, I tweeted BA (about the only reason I have a Twitter account...) to protect the LHR-SOF sector, having missed the JFK-LHR one. All sorted by the time I arrived in LHR. This was all one booking, though - separate bookings would rely on goodwill!