Clicking on the Diff Icon on the Changeset page or the Diff link on the activity stream is supposed to take you to the source tab for the file with the two revisions selected to show a diff.

For a move or copy, where the diff parent is at a different path, this page does not have the capability of diffing against another path, so errors out trying to look up the revision to diff against and gives a totally missable error message and shows the revision list instead.