I recently checked my stats and found lots of 404 errors on images in posts. I first noticed it with my background image but when investigating found many more also.
Here is an example of a link from the media library:
The above shows a preview of the image, but when clicking on it, it says it fails to load.
I have a small suspicion that this may have been caused by the Regen. Thumbnails plugin which I run if I ever add some new custom image sizes.
Has anyone experienced anything similar?