I suppose there is always a chance, but I frequently leave without formally checking out (because the TV won't let me check out after 11am at most hotels) and calling the front desk often takes forever. They have never charged me anything additional.

Back when I worked at a hotel, this was a fairly common practice & they would just send someone (either housekeeping or front desk) to check the room after it was cleaned to make sure the guest meant to check-out, before allowing new guests to check-in.