Rsync should reject an attempt to pass an argument to a command-line option that doesn't take an argument. For example, this incorrect attempt to disable --perms should be rejected:
$ rsync -a --perms=off ...
Currently it works just like --perms.
The above example was seen on the rsnapshot list:
I'd imagine that this is a bug for the popt library.
But don't bother trying to comment there: their bug tracker doesn't allow non-developers to comment. That's ridiculous. I complained about that too:
Created attachment 4974 [details]
Patch for rsync's included popt
For now, here's a patch applicable to rsync's included popt.
Created attachment 4975 [details]
Better patch (catch --quiet)
This is now fixed in rsync and upstream.