Mass Effect: Deception is a novel set in the Mass Effect universe. It chronicles Gillian Grayson's attempt to avenge her father's death at the hands of Cerberus in Mass Effect: Retribution. The novel contains many continuity errors, which BioWare has promised to correct in subsequent editions.
Mass Effect: Deception provides examples of the following tropes:
Artistic License - Biology: You cannot outgrow autism. You can learn to control it with time. You can use drugs that help control some of the symptoms. But you cannot outgrow it.
Badass: Kai Leng takes this to absurd levels. He kills the most powerful human biotic in the galaxy with a toothbrush. Keep in mind that Kai Leng is a completely normal human (at least relatively so) who just had both of his legs shot, and has to move around with a cane. Speaking of which, he also kills a guy with a jump kick, even though it's previously established that he's crippled.
Chronic Backstabbing Disorder: Kai Leng gets two mercenaries to help him steal Grayson's body. He traps one of them in a containment device to act as a decoy, prompting the other genre savvy merc to demand payment up front. He complies and kills that merc a few hours later.
Canon Discontinuity: At least the original editions, which a rewrite will replace in the continuity.
See here. Long story short, the novel makes many errors regarding the lore of the Mass Effect universe.
See the Internet Backdraft section on the YMMV page, but BioWare announced that a revision is coming to correct the many, many errors.
Cutting Off The Branches: The Citadel Council in Deception are made up of an asari, a turian, a salarian and a human. Thus, the playthrough where a Renegade Shepard sacrifices the Destiny Ascension, making Udina appoint an all-human Council (as opposed to a still-mixed-species Council for a Paragon Shepard) is invalidated. Mass Effect 3 later confirmed this, as there is no all-human Council in it, either, unless it abdicated between the games. Notably the human councilor is not identified as Udina despite Retribution implying otherwise, but his actions and personality identify him clearly as such. Nobody's sure why the author didn't name him.
Defeat Means Friendship: Despite their sole interaction being a one-sided conversation that ended with him being thrown across a room, Nick greets Gillian with open arms and a kiss on the cheek when he sees her again.
Downer Ending: Gillian, Nick, and Hendel all die pointless deaths, Kai Leng gets away again, the Illusive Man is building an army of Grayson clones (or something possibly worse) and once again the Council sticks their heads in the sand and ignores Kahlee and Anderson's evidence about the Reapers.
Evil Is Petty: Kai Leng plants a bug in Anderson's house. Before he leaves, however, Kai Lang eats Anderson's cereal.
Forgot About His Powers: After having used an elaborate sensor suite and multiple-member strike teams to accomplish his goals in previous novels, in this book, Kai Leng camps near a door with a vase to piss in so that he can keep a sniper rifle trained on it.
Idiot Ball: After already losing Nick on the Citadel, Kahlee decides it would be a good idea to let an emotionally unstable Gillian go outside for a spot of fresh air after she's announced her intentions to kill the Illusive Man. Three seconds later she's missing too. The novel does point out this was incredibly stupid thinking on Kahlee's part.
Aria uses next to none of the security she used in previous books/the second game. Luckily for her, Kai Leng has forgotten about the advanced tactics he used in previous novels as well.
I Gave My Word: After one of Aria T'Loak's banks is hit by the Grim Skulls, she lets one of their captured mercenaries go after the merc tells Aria who really killed her daughter. When Aria later confronts the same merc in the Grim Skulls' medical bay, she has her transferred to a hospital, her reason being this.
That Makes Me Feel Angry: A great deal of criticism has been focused upon the author's inability to write characters in a natural manner.
The Mole: Cory Kim, ex-partner to Kai Leng, is a deep undercover Ceberus operator in the Biotic Underground. Her cover story is that she quit Cerberus before joining up with the Underground, which even Kai Leng believed at first.
You Killed My Father: Gillian toward the Illusive Man, but there's a few more variations of this as well; Aria to Kai Leng over her daughter, Kai Leng to Gillian over his ex-lover, and Nick to Hendel over his girlfriend.