On Various Artist releases where there are enough discs for the advanced tracklist edit to show the discs collapsed when the page is first loaded (for example, releases with 5 discs), the artist field on the advanced tracklist editor is unchecked by default and disabled from editing without first enabling it.

To recreate:

Go to a many disc release (As an example: http://musicbrainz.org/release/90d4c5e9-5bd8-4801-ab00-ee0c250de4a1)
Click Edit
Click Tracklist tab
Click Advanced button
Expand a disc
Add 1 or more tracks
Note that the new tracks have 'Various Artists' listed as the Artist, and this can't be changed as expected by just clicking the artist field and typing. Instead (and this took me a little while to figure out), the box next to artist must be checked to allow the field to be edited.

Given that the field is automatically checked and editable straight away for VA releases with few discs (e.g. 2 disc releases) this is inconsistent, and makes for a frustrating user experience.