Yeah, it's happened enough times that I started wondering what caused it, and then I narrowed down the variables to what I saw above. I'm not sure if it can happen in multiplayer - I may set up a LAN game between two computers and test it out.
Next up: Bribing a fortress!
Now, most people know already that you can bribe any enemy building that's in your territory. This usually happens when you have captured a city or built a fortress and your border pushes so it includes the enemy buildings.
However, it's usually not possible to bribe fortresses, because they usually have their own border push. This usually means they never end up in your territory.
The exception to this rule is when they're being built. If an enemy building is under construction and ends up in your territory by border push, you can bribe it and indeed you can even finish building it.
Here Blue has begun building a Fortress near the border. Red's border push has forced the Fortress site into its territory. Note the dead Blue Citizen from attrition.
Now Red has bribed the worksite and is the legal owner of the unfinished Fortress. Note the Red Spy admiring her handiwork.