I did it once with NZ on a F-RTW to catch a flight just 2 hours earlier. They messed it up (insisting on reissue but without fee, change of PNR+tix number) completely. It took me days to sort that out for the rest of the itin afterwards and of course an additional 30 min at check-in before every single following flight on that itin ("Sir, you are not booked on that flight").
Lesson learned.