If you're typing a private message to send to someone and haven't put in a title and click on the preview button, the preview pop-up window comes up but it's blank.
It's blank because user won't be able to post the message anyway and it indicates there's something wrong. I think it's a good and accessible feature.
Obviously there could be different scenarios like missing title or title too short or to long, missing message body, message too short or too long, etc. - the blank page covers it all and makes the code nice and clean :).