This morning I was doing a tip mission where you need to save Polar Shift from Longbow. I've done the mission many times, and never had a problem. This tells me never be complacent!
The first problem is that even though Polar Shift and her Longbow captors are on the second floor, you 'hear' the boss talking to you while you're on the first. Again, strange, but normally not an issue. Today when I went up to the second floor, I could not find Polar shift. In fact, I ended up resetting the mission twice, because she wasn't there.
Then I found out why. Seemed that one of her Longbow captors was a spine character, who was constantly shooting spikes into her, while she stood there being impaled until she died. So, when you get to her, she's already been gone for a bit.