This would occur in cases where an image failed to load and the alternate text is displayed in place by your browser.
There are various causes: A temporary flakey internet connection between you and our image server, or any other kind of momentary glitch on a server along the way. It’s also possible that if you’re viewing a Tweet that includes another user’s image as a link, and that original Tweet were to have subsequently been deleted, that that raw image would be unavailable permanently (although Tweets like these are rare.)
Ben
