Well in Civ 5 if you build a unit, and there is already a unit in the city, you can't end your turn until you move one of the units out of the city.
I assume you also can't end your turn if you have nothing in the queue.
So if you are producing a Scout (25 hammers) and are producing 90 hammers per turn.
on the next turn you have
1 Scout, +65 Hammers overflow
you have to put something else in the queue...let's say you put in another scout
...it gets insta built
2 Scouts, +40 Hammers overflow
you have to put something else in the queue...let's say you put in another scout
...it can't get built until you move one of the two scouts out of the city. (which you HAVE to do before the turn is up...as soon as you move one of the two scouts out of the city the 3rd scout is instabuilt
1 Scout out of city, 2 Scouts in city, +15 Hammers Overflow
THen I decide I'm sick of Scouts and want to build a Granary...
so I have
1 Scout out of city, 2 Scouts in city, and a Granary that has 15 hammers complete
I still need to move one more of the scouts out of the city to end my turn.
I really hope this is how they Handle it.