This is a bug which I know about but haven't analyzed yet. If your redirect takes a long time or doesn't happen, open a new tab or window and make sure your thing has posted; if so, you're safe to move away from the submission page. Normally this would happen automatically, and still does sometimes, but not always.

Working on a resolution.

Edit: This is fixed. Do not run a reddit installation with memcached 1.4.3; there is a bug in memcached that makes it very, very slow, at least with reddit.