Particularly zealous Zotero users will occasionally push the limits of what Google Scholar, or any other Zotero-compatible database, wants an individual to access in a given period of time, and you may see a message like the following:
“Query limit reached”
“We're sorry but your computer or network may be sending automated queries. To protect our users, we can't process your request right now.”
This tends to happen if one tries to import 50-100 items from Google Scholar either all at once or in rapid succession. If you need to import such large numbers of items, it is better to first save the items to Google Scholar's "My Library" feature, then export items en masse as a BibTeX file from the My Library view.
The Retrieve PDF Metadata function also uses Google Scholar, so, if you want to auto-detect metadata for several dozen PDFs, try doing them in smaller batches with a few minutes in between each set.
Sometimes a site will request a CAPTCHA verification. In this case, complete the verification test to restore access (if Zotero encounters a CAPTCHA request using the Retrieve PDF Metadata function, it will prompt the user). 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.