Back in 2001 I flew from LHR-TLV. Got a message on my mobile on the day of departure (quite possibly left much earlier - I'd only arrived back home that day) telling me that the flight would land 2 hours later than scheduled (but departure was fine). I didn't understand how this was possible (turned out we were changing crew in Cyprus), but since the only 'problem' it caused was my brother having to meet me at 6am instead of 4am, that was fine.

Return flight was the next day. My taxi turned up half an hour earlier than booked, and we got to the airport 40 minutes earlier than planned, and 40 minutes before take off. The flight had been moved forward two hours, as part of the same rescheduling which involved the outbound landing in Cyprus. Fortunately I had nothing to checkin, and the initial security check where they ask you lots of questions took about 30 seconds, otherwise I'd have been stuffed, though I suppose BA would have got me on the night flight. Who knows why they only told me about half of the rescheduling, but I didn't complain much - I got to stay in bed for an extra two hours, and I made my flight.