When saving multiple items, Zotero users will occasionally push the limits of what Google Scholar, or any other Zotero-compatible database (e.g., ProQuest), wants an individual to access in a given period of time. When this happens, saving to Zotero may fail, and when browsing the site you may start seeing CAPTCHAs or messages about making automated requests.

If you need to import more than a few items at once, it's better to first save the items to Google Scholar's "My Library" feature, export items en masse as a BibTeX file from the My Library view, and then import the BibTeX file into Zotero. Other sites may have similar features that allow exporting a collection of items. You can also simply click through to the article page and save from there rather than saving from the search results page.

The Google Scholar Citations for Zotero plugin can also send sufficient numbers of queries to cause Google Scholar to lock out Zotero.

Sometimes a site will request a CAPTCHA verification. In this case, complete the verification test to restore access. Otherwise, in general, the site will restore your access within a few hours. You may be able to access the site using a different web browser in the interim.

If you're still able to access the site in your browser but not able to save to Zotero, as a temporary solution you may be able to save to BibTeX or RIS, as explained above, and import the file into Zotero.