Author's note: because I'm not yet allowed to post links in the forums, please note that all links havedot for the actual . in links, and any "http://" is spaced out so things don't appear as links. Common sense, folks.
I found the answer here: http:// wwwdoteightforumsdotcom/tutorials/13200-windows-8-upgrade-iso-redownload.html, but here's the brief summary:
-I'm guessing that at some point, you probably downloaded the Windows 8 preview, yes? For whatever reason, when you go to do it "for real," the Windows 8 setup doesn't overwrite the old file, called the "WebSetup" folder. If that folder is still there, you'll get the "Sorry, something happened and we couldn't finish creating the ISO. Restart setup and try again" message.
The location of the folder is at: C:\%UserProfile%\AppData\Local\Microsoft\WebSetup
where %UserProfile% is the user account you downloaded it under (probably your default account) - this is the folder you need to delete.
Once that folder is deleted, run the Windows 8 setup file (found here: http:// windowsdotmicrosoftdotcom/en-US/windows-8/upgrade-product-key-only if you haven't, like me, already downloaded it 8 different times thinking it was a corrupted file), and it should prompt you at that point for your product key - which is something it wasn't doing for me. After that, you can choose to make it an ISO, run it normally, run it from USB, etc.
It'd be helpful if Microsoft knew about this... hope it helps...
(h/t to EightForums, where I originally found and condensed this guide.)
