If the file was added and updated later then **Show Raw File (Left)** command is missing for it in diff of diffs. Repro is [[ https://secure.phabricator.com/D2172?vs=3709&id=3710#0972ae59 | D2172?vs=3709&id=3710]].
The similar problem will probably appear for other edge cases (e.g. file deleted in second update).
The fix would be probably one-liner, I just couldn't find this line...