What is "url signature expired" and why do I keep seeing it? In the past I have only seen it when I click on some .mp4's, but lately I have been getting it on some .jpg's, too.
You'll see this with Facebook links. Their url's for users' pics/vids have a "shelf life" and will expire over time. Not sure what the exact expiration time is, but if you check some links from posts made early this yr, you'll see all/most have expired. It might also be caused by people deleting their pics in response to harassment from the creeps.
That's why we've always encouraged our members to save their finds, and post them as attachments rather than links.