If the final layout is finished and an image hasn't finished loading yet, a grey box saves as a placeholder.
The border extends beyond the image's area, and when the image is finally loaded, the part of the border
that's outside the image just stays there.
To reproduce you need an image that takes long to load (and that isn't already on your cache). You can go
to http://g4.mitzpettel.com/seeTheBorder2.html (until I take it down).
Expected: grey placeholder frame to go away once the image is fully loaded
Actual: gray frame remains. If the page is reloaded (from cache), the frame disappears
I'm almost certain that this regression started with the patch for bug 4924
Created attachment 3893 [details]
Created attachment 3894 [details]
Got this fixed.