529 Site is overloaded
Meaning
A non-standard code used by certain services (e.g., Qualys SSL Labs API) to indicate that the site (or service) is temporarily overloaded and cannot process the request.
When to Use
When a service has a custom mechanism to signal overload separate from the standard 503. Qualys’s SSL Labs API, for example, uses 529 to tell clients that the server is busy or over capacity.
Example
Querying the SSL Labs analysis API during peak times might result in a 529 Site is overloaded, meaning the service cannot handle the request at the moment and the client should try again later.
When Not to Use
Do not use in general implementations as it’s not an official status code. For typical overload situations, 503 Service Unavailable (potentially with Retry-After) should be used. 529 is very specific to certain providers’ conventions.
Source
Unofficial (Qualys SSL Labs)