As we know, when you first go to a custom domain for a subreddit, you go through a redirect to log you in (when you're already logged into reddit, that is). If you skip going to the domain directly (in my case aprsn.info) and go to /submit (eg: aprsn.info/submit), this prevents you from actually submitting. You're sent to /login?dest=%2Fsubmit . While it is the submit page, the form does not work.
I'm using opera 11.60, if that's relevant. Sorry if this is hard to read, I no good at computer.
Custom subdomains have been turned into a regular redirect, so this shouldn't happen any more.