First of all, I'm not sure if it's an old bug; however, I may have something new to say about the possible same bug while it is still unsolved/unresolved/not going to solve...lol.
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At the beginning, you can blink to an unclaimed base, type -claim to claim the base, ... the magic is that you have to type -exile [your color] AND type -claim again where the screen message would show that you cannot claim an additional base, type -unclaim, and you have chance to have those units not from another bases you will claim after (those won't return to dark green fortress survivors).
Basically: -claim, -exile, -claim, -unclaim.
I did a few tests, but I couldn't find the exact cause. The easiest base that I have found so far is the security room, but it doesn't guarantee success every time.
I tried to rule out triggers:
1. It can happen on any mode and difficulty
2. It can happen in bases without quest
3. It can happen in bases without building
4. Bases that I successfully claimed extra units: Security Room, Harbor, Military Base, Shinkle's Brewery, Courtyard, Temple of Boom
5. If you failed to use this bug at the same place and in the same game, more than 5 attempts at same base wouldn't help
6. I was able to claim units from 3 additional bases max so far in same game
7. After you typed -exile and tried to reclaim the same base, there are chances that only partial units can be reclaimed.
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Originally, I was wondering how long I can survive without any building with the help from survivors...anyways.
The second bug is the Quest in Security Room where you have to protect the tressure vault and Mr.... "Sanders" (not sure about spelling, just call it THE OWNER OF TRESSURE?). If you type -exile in Security Room, the Vault would disappear... and under the "F9" information section would show that the quest is still going whereas the Quest to protect Military Base would fail after towers are destroyed. (I thought vault is a building, but if I do not remember wrong...medic can heal the vault?...anyways, it's gone after -exile)
So, the advantage/bug is that you can "steal" units from security room > accept the quest> bring "the guy" to your preferred base, and enjoy the free super larceny as reward?... . . . Even if someone is bored enough to try to steal your stolen units by claiming bases, there are chance that "the guy" will remain yours, or "someone" has to steal your "the guy" and kill that unit before you get your reward by failing the quest...
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It turned out that the second bug doesn't exist, because monsters can still destroy the "invisible Vault"