The issue is this: when you click submit after writing a post or a PM, you often get an Internal Server Error, but when you go back to the thread, or go to your outbox folder, the message turns out to have gone through just fine. However, if you refresh, or go back and press submit again, the message is posted or sent twice.
Does this sound familiar? Please answer the poll.
Edit: due to another technical issue, this thread is locked for most members. To vote in the poll, or submit a reply, please refer to this topic.