Simon Cowell in the Town
As much as OPTIMO APIs are quite capable and fun to be used, they sometimes can be rude if you annoy them and they act accordingly, much like Simon Cowell buzzing off contestants that he does not appreciate in Britain's Got Talent! Clearly APIs do not have buzzers; instead they throw Responses ("errors" would have been perhaps a better terminology, but let's not go on that path). There are different kinds of responses depending on how you annoy them, and each kind has a certain three-digit code so that they are standard across all APIs around.
The descriptions of possible responses are listed under each endpoint in the API Reference. Few examples may be useful here.
You receive these Responses within the errors segment at the end of the JSON response. For instance, if the access token in your API request has expired, then you receive the following error in the JSON response.