As per install instructions I ran the macosx_setup.command to setup arduino (USB version) on OS X. It failed with an error message (something about /groups..... ; don't remember exactly sorry). Then I tried to upload a sketch with Processing which failed also (Unknown application error).
I quit Processing and reran macosx_setup.command. No failures this time. I re-uploaded the sketch, this went fine too.
Maybe this should be clarified in the installation instructions?