I noticed this today while putting together a quick and dirty tool to do a search on all wikis (using OAuth mostly because I could reuse my class). When the script came to certain wikis it wasn't getting a legitimate response back which at first I thought was just intermittent or I wasn't skipping closed wikis correctly.
After some troubleshooting I realized I was getting a '500 Internal Server Error' message back instead for those wikis and after checking a couple realized it was the 2 dozen or so wikis where I didn't yet have a local account. Obviously fixing that was easy (I clicked over to those wikis, my account auto created, and I ran the search again) this doesn't seem to be how we want the process to go.
For closed/fishbowl/private wikis I get a relatively nice json response saying that I don't have an account there which works well. I guess, at least, sending a message like that here would be nice. I would prefer, however, that it just auto created the account as if I was 'visiting' for the first time since I kind of am just in the form of my oauth token. My working assumption (based on no real information and just on the symptoms) is that the wiki doesn't return a 'no account found' error because it hits a snag when it finds my oAuth information in the centralWiki but doesn't find my local user account and so 'freaks out' and throws an error.