The bug exists within ditto, the utility that BOMArchiver (the graphical front-end for zip extraction on OS X) uses.  The error reporting on the Console is "Couldn't read pkzip signature."  A quick investigation reveals that this occurs when the data descriptor flag is set (see However, a quick overview of Fossil's zip files doesn't appear to indicate that this bit <i>is</i> set, so I'm at a loss.