With Schengen an international flight between two countries both in the schengen agreement is treated like a Domestic flight, i.e. no passport control/Immigration, only Photo ID is needed for travel.
Finnair have a flight that operates HEL-ARN-BOS.
This flight carries local pax HEL-ARN, as well as pax HEL-BOS.
To travel between HEL and ARN you only need an ID card as both are Schengen. But as this flight is going to the US, it departs from the Non-Schengen area at HEL. To get into this area you must have a passport. This is pointed out at the time of booking, but not everyone reads the small print, and pax are denied travel.