In the first scene, it shows the rack of violins have collapsed. In the following shot, when Trish enters the store, you can see the bottom rack of the violins shelf isn't collapsed, but it is possible that they fixed it off camera.
In the scene before Nelson enters the store with the piano, you can see three music stands, different colored, standing by the cymbals.
After Nelson leaves, the three music stands get moved but no one seemed to have touched it.
During the scene where Ally is talking about Dallas, there is a camera on a tripod, which means it has to stay still. But when it shows the Jumbotron, the camera seems to be moving but it shouldn't move since it is mounted on a tripod. (You need to see the video from the episode in order to watch the moving camera on the jumbotron.)
When Austin is performing "Not A Love Song" Ally is nowhere to be seen in the audience at first. Then when the camera moves to Austin then to Ally she has reappeared standing next to Trish as if she had been there the whole time.
Dez is wearing clown shoes but for the rest of the day he is not.