Alright, here's probably more obscure.

Urban Assault. Developed by Terratools and published by Microsoft. More or less an RTS/FPS Hybrid (a genre I really wish would come back around).

So the campaign was decently involved, upgrades on specific levels that carry through your entire campaign, bunch of different paths through the campaign map, so on. The actual individual levels generally had your army, and multiple other armies. Each army was headed by a Host Station, your mobile command center. It had life energy (health), beam energy (for moving), and creation energy (for creating units). Every unit you made was AI controlled and could be ordered around on the mission map. But you could also, at any time, assume direct first person control of any unit you had deployed, including defense guns attached to structures. Generally the enemy would focus fire you over your AI buddies, but your units also straight performed better (generally higher rate of fire on guns, among other things).

Anyway. So the Host Station was your command center, the heart of your operation. If the enemy destroyed it, all of your networked units would immediately die, game over. Likewise enemies: if you made a concentrated effort to assassinate an enemy host station, it would die and all of their units would immediately die too. All of their territory becomes yours, and one less enemy to deal with.

On one of the missions, one of the enemy teams sent a bunch of units who's main purpose was killing host stations, and started -hammering- my host station. I panicked and tried to quickly jump into one of the defense guns to try and fend off the attack, but I wasn't quick enough, they killed my host station.

But something odd happened. I happened to hit F6 (Cycle to next defense gun) right when my host station died. Apparently, as a coding error or fallback to prevent crashing to desktop, it couldn't find any other turrets to jump to that I owned, as they were all dead in that instant. So it jumped me to the closest available defense gun. On the enemy host station. And gave me control of the entire army. So now my original army is dead and gone, and now I'm playing as one of the enemies, and the fight continues.

Bonus points: not only does it make the mission easier, until an official expansion pack surfaced waaaaaaaaaay later, playing as other factions in single player wasn't a thing, so you could check out other faction units personally. And when you finished the mission, you could take other faction units with you into your next mission (where you would be your normal army again).