I have never seen that. I usually get bumped to Business between ARN-IST though if Im on an economy ticket. I would think they don't overbook more than economy up to available seats on the whole plane.

I have been on one flight where one person in Economy was seated in a blocked middle seat in business, but onlöy once.

I guess it might change now that TK has decided to start flying larger A321 for all flights between ARN-IST now.