First, what happened was that I suddenly had a new city appear out of nowhere on a tile where one of my random units was. The message was that the unit captured the city. That's all the info I got. The only thing I could imagine that could've happened is that one of the factions that had open borders with me (Togra, basically), decided to try building a city in a tile occupied by my units, and since the newly-built city was empty and my units were in it, it counted as me capturing it. Any chance that's what happened? It'd make sense if I was at war, but not when anyone who could've built it is friendly towards me. (Although, the next incident kinda has the opposite result.)
Second thing that happened was a few tiles away from the above incident, one of my fort tiles was suddenly a Togra city. I scrolled there just in time to see my units that were in the fort being pushed outside. Can another faction's colonizer build a city in a tile that has 1. a fort in it, 2. is under your city's control, and 3. even has your units in it? That's taking Open Borders a little too far me thinks.
[EDIT]
Here's a screenshot from a save a little later on in the same game.
* Hubble is the forest tile that was under the control of Smuts, and had my 'Scout Mk.I' infantry in it. Then Hubble appeared out of nowhere. Scrolled the log to find the message about capturing it.
* Lord Todd is where I had a fort, also under Smuts' control, as well as couple of infantry units. Then it was suddenly a Togra city.
- pandorabeta_weirdstuff.jpg (250.01 KiB) Viewed 1561 times
