We have been having some issues with spam and abuse lately, so have had to impose an rate limit on our anonymous web API. If you are using the web API ensure you set your application id.

See,

Just a note on how to correctly use the web API.

When you use the web API ensure that you pass your application id. An app id is free, and you can easily get one from your user page. You need to pass your app id on every request.