In my experience, when I tried to board the wrong flight, the scanner served its purpose making an alerting sound and making sure they'd prevent me from boarding on the wrong airplane.
This was with Delta (both my intended flight and the mistaken one.)
Delta's IT is pretty decent for a large airline. Their scanners have little built in receipt printers. If you scan the wrong boarding pass (e.g. first leg of your flight boarding the second leg), it'll make the connection and pop out a little receipt telling you where to go. If your seat gets reassigned at the last minute or you get upgraded, you get a receipt giving your seat number.
This was with Delta (both my intended flight and the mistaken one.)