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.
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.