Because you can make 'gaps' in a city built a certain way, spaces that are valid for units to eject into, this happened:

That scout can't get out of the city because the city's garrison is full, so it's not allowed to travel through the city.

To fix it, you have to move a unit out of the city, then move the scout, then put the garrison back in, or put the scout in the city, select it, and move it out manually, since V-ejection removes everyone and dumps them on a random square.

Messy all around.