Lines starting with '---', '+++' aren't necessarily diff control lines.
Review Request #2357 — Created May 9, 2011 and submitted
We ran into this bug with a file containing a database schema. It was well commented, so a lot of lines started with '-- ' followed with comment text. Unfortunately, when these lines were deleted the resulting raw diff contained '--- ', and as the raw diff was processed those lines would hit this code.
We were able to submit a properly-formed diff of the schema file to ReviewBoard with this patch applied. Without it, the diff was malformed.