Nope, not a Halloween joke, Canon has confirmed a glitch in the rapid firing of the Canon EOS 7D which may produce ghosting (a residual image from a previous image). We first saw this on canonrumors a few days ago. A bit easier to show an example than to explain…
From this post on DPReview where there's more discussion and a few other samples (note, the photographer of this image is not identified). This image has been enhanced and outlined to show an extreme case of the ghosting.
Canon describes it in their Service Notice for the Canon EOS 7D like this:
In images captured by continuous shooting, and under certain conditions, barely noticeable traces of the immediately preceding frame may be visible. This phenomenon is not noticeable in an image with optimal exposure. The phenomenon may become more noticeable if a retouching process such as level compensation is applied to emphasize the image.”
When a fix is released, we'll be sure to let you know!
(Photo credit: snap from the DPReview forum post)