Being 'moved' to another flight can always cause a problem with the original flight.

I think the old-timers word for the screw-up is that an 'arunk' was not placed (or misplaced) in the record.

Arunk I was told stands for 'arrival unknown' and is some kind of little computer symbol that has to be placed in the reservation properly.

Similar situation .. SJC-HNL delayed 5+ hours (would it even go?) so I was placed on a SFO-HNL flight the next day as 'backup'. Eventually the SJC flight did leave ....

Got to HNL for my return and my res had been cancelled since I 'missed' the original flight. Gee how did I get to HNL then in the first place if I did not fly somehow??

That was when I heard about 'arunk'... and that some agents when they protect you or volunteer you on flights do not properly place this old/new record things can get VERY messed up.

What I have done since then in any similar situation is to ask the agent (very nicely) if they have placed the 'arunk' in the proper place so I would not be cancelled on my return. They all keep saying 'yes I have done it properly'.

I guess the other advice (yes being proactive) would be to always check the return flight the return/continuing flights (phone vs internet) the following day (the 'no show' program probably runs overnight).