Over in Delta-land, this happens regularly on (for example) the continuing Atlanta-Sydney routes that stop in Los Angeles. The 777 used for DL17 between ATL-LAX is not actually the same plane that continues LAX-SYD - the transpac aircraft currently comes in from DTW.

I've heard they designate the ATL-LAX flight 17A when it is running significantly delayed to differentiate it from the (possibly on time) LAX-SYD departure... no data to back this up but I do recall it coming up in a discussion previously, and it matches with what some others have said so far in this thread.