Wasn't paying attention to my suits power, and my cloak deactivated while I was swimming across the river. The attack helicopter overhead, already pissed due to me trying to blow it apart with a rocket launcher earlier, decided to have a bit of revenge. I tried diving underneath the water but it wasn't deep enough to offer any protection.
It's sad that, years later, I still know the mission you're talking about. Always felt like that river was way too long, but maybe I missed something en route while running from shrub to shrub.
On-topic: brought a knife to a gunfight. That alone wasn't a problem, but sadly the knife was modded with Blast damage, which did absolutely nothing to the Protectors. The first couple were electric, and two of my teammates had electric abilities, so all was well. The third was Cold, however, and the combination of his aura and our team's complete lack of frost damage caused a team wipe.
Warframe. Protectors are a Grineer unit that's immune to all but one basic elemental type, and they extend that invulnerability to nearby allies as well. I completely forgot the mission I was undertaking was Grineer Survival and didn't mod my weapons appropriately. Doh.