The Teleric Bookvar ad image is broken on Stack Overflow.
Scroll all of the way to the bottom to see the broken one.
It's not broken in the answer.
It's still broken as of today.
instead of the ad image you expect:
Apparently they're watching for broken/non-existent images and replacing them with a generic SO ad (which is a good idea!), but apparently it only works when the URL has a normal file extension.
According to someone at Telerik, the file extension
So it looks like whatever process transfers an ad to the SO rads server once it has enough votes wasn't able to retrieve the image from the redirector file, and the default ad isn't showing up because of the weird, non-image file extension.
1 I also learned that SFLB is an acronym for "Short in the Front, Long in the Back", aka the mullet.