Technically it's supposed to void it (from my understanding)
BUT
Many MANY times I have seen the checkin Agents at YQF tell someone to drive to YYC when they are doing YQF - YYC - Something else, and they have been allowed on.
note: this happens because at this super super super small airport (maybe 200sq feet after security and 100 sq feet pre security) the 45 minute rule still applies, and many pax don't show up early enough and the gate agent refuses to sign them in and tells them to drive to YYC - seen it dozens and dozens of times.