The master mirrors may oddly have the setgid bit set on various directories. There isn't really any use for this on downstream mirrors, and it can cause issues when mirroring to a filesystem which doesn't allow the setgid bit to ever be set without elevated permissions (like AFS).

Adding --chmod=Dg-s to the command should help with this. Add it by default, and add an option SETGIDDIR to turn it off.