Someone kept mentioning how often the word nightmare is used in the game, so I threw together a collection of nightmare quotes from the vanilla game's tlk files. Enjoy.
Mission Tuchanka where you meet the Primarch's son
"The crash site's a nightmare."
Well the conduit run is sure as hell a crash site.
Upon turning and seeing a Reaper capital ship on Thessia,
EDI: I am starting to understand what the word "nightmare" means to organics.
EDI: And why you feel trapped when you can't wake up.
Tali: As a child, I always had nightmares about geth attacking me in my sleep.
Tali: The worst horror I could imagine. Until now.
Later on Thessia
Liara: This nightmare never ends.
Shepard: The hell it won't. We get to this artifact, and we can all wake up.
Shepard: What did you say to the Illusive Man?
Dr. Archer: I told him if his intention was to work with the devil, he only had to look in the mirror. I wanted out of his nightmare.
Out of TIM's nightmare, huh?
Dr. Archer (pulling a gun before killing himself with it): A precaution if I was ever captured by the enemy. Or couldn't wake from my nightmare.
Dr. Archer: I couldn't bear the thought of reliving the nightmare. So I destroyed all of my research--Overlord is no more.
Dr. Archer (Explaining Overlord to a Shepard that never played it): Believe me, it's a nightmare I re-live every night. He went berserk--his mind became a computer virus that infected all our systems.
Damn, Archer has a lot of "nightmare" dialogue. I'm sure it's just a coincidence and nothing to do with his synthesis-control stuff.
Shepard (to Vega): I can hardly believe it, myself. Like everything back on Earth was some kind of nightmare.
Turian Soldier: It's a shame to see another planet get hit like this.
Garrus: It'll be the last one if I have anything to say about it. This nightmare finally ends today.
Liara (encounting ravagers, Reaperized rachni, for the first time on Tuchanka): Now I can have all-new nightmares about them.
Unknown, at Ardat-Yakshi monastery: Now it's mutated asari. The Reapers are just a giant nightmare factory that never ends.
Shepard and Liara chatting on the Normandy:
"Too bad we never found another Prothean beacon."
"Haunting the Council with visions of the Reapers might've saved us a few years."
"Well, anyone who lives through this war won't be short on nightmares."
Garrus: Some of the crew seems shocked by the monstrosities we have encountered.
Javik: They haven't seen what the Reapers could corrupt after a hundred years. That was our war. Every battle conjured a new nightmare.
Unknown, after Rachni queen mission:
"I still have slime to clean off my armor."
"What a nightmare."
"Dark tunnels, toxic goo, and those damn cockroaches again?"
Unknown, after Rachni queen mission (Wrex or Wreav?): Sounds like it was a nightmare down there.
Allers doing a live report: Breaking now: A nightmare on Earth. Human leaders using military force on their own people.