I got bitten by this:
Now, obviously that's just my bad luck. Spent a while fruitlessly Googling - it's easy to find a couple of pages on this, but only after you know what the problem is; the symptoms alone make for less specific search terms. Then spent another while building various extra debug lines into the source to finally work out what was going on. I suspect a percentage of rsync users might get bitten too and would have no idea how to debug it (or would know how but wouldn't try).
How about a sentence something like this within the docs for -g and/or -o?
Note: on Mac OS X removable volumes the option "Ignore permissions on this
volume" can interfere with uid/gid comparisons, potentially causing false
> Note: on Mac OS X removable volumes the option "Ignore permissions on this
> volume" can interfere with uid/gid comparisons, potentially causing false
Whoops, that should be "Ignore ownership on this volume".
Added a mention to the manpage for 3.0.4.