Originally Posted by
krayZpaving
He may have a problem with using the return leg, if that's his intention, if he either doesn't complete the outward leg or doesn't start the return at LGA.
If he's just planning to get off at YYZ/YUL and dump the rest of the ticket, then it's not a problem.
Any onward travel after the missed segment will be cancelled and its value lost. May also cause OP some serious issues in future admission to USA because the manifests are pre-cleared on USA-bound flights. Lastly, in IRROPS, the carrier will fly OP to his destination (LGA, JFK, ORD), not necessarily on the route on which he's booked.