StarCraft® II

It says I haven't talked to Dr. Ariel Hanson but I am 100% sure I have because I talk to every character in every room after every mission. And now I can't go back and talk to her because she turned into an infested terran and died.

SPOILER ALERT

/golfclap

How about warning the readers that you are spoiling the plot? >:(

Go into the mission archives and complete the alternate mission you could have done (instead of killing all survivors, rescue them) to unlock the achievement.

I don't know about Decapitator's suggestion, but one thing that will work is to start a new campaign, and talk to her in that one. This achievement doesn't track that you're doing all of the bits in the same campaign, so you'll be fine. (Make sure to save your current game first)

You only need to do 4 missions before she shows up to talk to, which should be really quick on casual.

(I needed to do this because I did her plot-line while achievements were down last week - it works)