WorldCat?
Attempts to gather metadata for an individual item on the WorldCat webpage via the Zotero connector in Firefox have been regularly failing for a couple of weeks now (the "Saving to"-panel appears, persists for a while, and eventually either disappears or I close it; nothing is saved). I've tried the things on the connector issues page, with no effect. Debug ID 406801.
(When working properly, as it does for me, it will still show a fallback error in the save popup, but it will then save successfully. I'm not sure why that's necessary, but someone more familiar with our two WorldCat translators would know more. Details here.)