The second scene in Professor Starkman's class takes place two days after the first one yet everyone is wearing the same clothing and the word "Nicknames!" is still the only thing written on the blackboard.
The goof item below may give away important plot points.
This movie is about a girl who kills Patrick Bateman during one of his killings. But the ending of the original American Psycho deliberately leaves open the option that the killings in reality never happened and that Bateman was just crazy. (Although if this is the chosen point of view, the very end of the movie does pay homage to the original with Kunis saying something along the lines of "It's like you didn't do anything at all".)