Add or import a VA release, e.g. http://test.musicbrainz.org/freedb/misc/9410e40b

Set artist (to Various Artists)
Click straight through to Edit Notes.
Add Missing Entities appears. select an artist for all but one option (I forgot to enter sort name for Unknown-NKS - hard to see where one artists starts and another ends on this page, but that's another ticket...)

When the page reloads (with error message), you've lost all of your selections. Very frustrating have to click them all again.