It looks like the problem occurs with .gifs that are recognized as .pngs
For example:
Back to the Future Part II
If you try to save the 8th picture you'll see a file named:
Back to the Future Part II_8.gif.png
However .jpgs pictures are ok, same for animated gifs.
It would be great to have a detailed list, but it will probably take lot of time for you to prepare.