Zotero uses “translators” to allow you to save items from the websites you visit. When you get the “An error occurred while saving this item” message while trying to save an item, first check the list of known translator issues below. If the translator you've been using is already listed, you don't need to take any action (unless you have some programming experience and want to help fix the translator). Otherwise, try going through the steps for troubleshooting translator issues.
You can see which translator Zotero tries to use by hovering your cursor over the “Save to Zotero” icon in the address bar of your browser. The tooltip shows the translator name between parentheses.