This mistake is right near the beggining when you first see Andy as an Adult.

After he sings the Animal Song, he then does impressions of Jimmy Carter, and then Elvis Presley.

While he is getting dressed into his Elvis costume, notice that he puts a guitar around his neck.

After he's all ready the camera views the back of Andy as he walks up to the microphone. Then when the camera shows you the front of him, his guitar has suddenly dissapeared.