It's the one that massively contradicts canon, the one where Gillian suddenly doesn't have autism any more, volus wear open masks instead of sealed environmental suits (which would result in their bodies splitting apart from the pressure difference), batarians snatching slaves from the turian homeworld Palaven, and Aria T'loak suddenly having family members named T'loak despite "Aria T'loak" being a pseudonym.
I haven't read it myself, but the online reaction to it is delicious. And, being a Mass Effect fan myself, I do agree that such errors are pretty bad.