To reproduce:

• Edit a release with 1 medium, go to the tracklist tab
• Add a second medium
• Remove the first medium
• Go to the edit note tab and attempt to submit the edits

Expected behaviour: The new medium becomes medium 1, and since medium positions must be unique, the medium being removed becomes the last medium (in this case, medium 2).
Actual behaviour: The release editor submits the edits to add and remove the mediums, but fails on the reorder mediums edit with "Bad Request: 400" (which is an internal server error behind the scenes) because it only tries to move the new medium to position 1, which it can't because the medium being removed is already position 1.