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Gary Illyes from Google posted on Google+ that he sees many large sites lose a ton of their pages in the Google index for misusing 403 status codes when they should be using 503s.
He said:
We've seen huge sites lose load of their pages from our index because they were serving them with a 403 status code instead of a 503.
A 403 status code will pretty quickly remove your content from Google's index. A 403 stands for a forbidden code:
The server understood the request, but is refusing to fulfill it. Authorization will not help and the request SHOULD NOT be repeated. If the request method was not HEAD and the server wishes to make public why the request has not been fulfilled, it SHOULD describe the reason for the refusal in the entity. If the server does not wish to make this information available to the client, the status code 404 (Not Found) can be used instead.
While a 503 status code is more temporary and normally doesn't lead to immediate removal from the Google index. A 503 stands for a Service Unavailable code:
The server is currently unable to handle the request due to a temporary overloading or maintenance of the server. The implication is that this is a temporary condition which will be alleviated after some delay. If known, the length of the delay MAY be indicated in a Retry-After header. If no Retry-After is given, the client SHOULD handle the response as it would for a 500 response.
So don't make mistakes with your server status code, it can be seriously damaging to your Google traffic. Unless you want to try the 418 status code ;-).
Forum discussion at Google+.
Image credit to BigStockPhoto for 404 error
