When viewing the vault, and scrolling between summons in full-screen view using the keyboard arrow keys, it doesn't properly update the "Save to Disk" button - it will save the image with the correct filename but with the image of the summon you originally clicked to enter full-screen view, not the one you are currently viewing. If you use the mouse to click the left or right arrow on the screen instead it works correctly.
(Also it would be nice to have it remember the last saved-to directory rather than always defaulting to the location of the exe, but that's a feature request not a bug)