When Marshall looks in the mirror and sees himself as a gorilla, the shot over his shoulder looking at the mirror shows part of him in the foreground. You can see him wearing a gorilla mask as he looks in the mirror.
During Barney and Marshall's discussion about the penguin poster, the clock behind Barney shows inconsistent times.
While in Germany, Victoria is supposed to call Ted at "11pm". Due to the time difference between Germany and NYC that would make it 5am in Germany, and thus a highly unlikely time for her to call.
When Ted goes to the karaoke bar, he sits down to talk to Marshall. Barney is heard singing "Dirty Deeds Done Dirt Cheap" by AC/DC in the background while Ted and Marshall are talking.
However, at points in their conversation, Barney can be heard singing into the microphone when he is actually dancing with the microphone nowhere near his mouth.