What is the difference between a 403 and a 429 error from Google?

A 403 'Forbidden' error means you're explicitly denied access to a resource. A 429 'Too Many Requests' error means you've exceeded a rate limit. While both prevent access, the 403 is about *permission*, and the 429 is about *frequency*. If you're hitting a 429, you need to slow down your requests; for a 403, you need to address permission issues.