Question:
Your client, once again, complains they receive a 403 Forbidden error when trying to access your API. It was working just 5 minutes ago, they claim. You look into this "urgent" problem and see no policy changes have been made, and surprise, surprise, it works on your machine(tm). How would you make client to shut up and be happy?
Answer:
A 403 Forbidden error is either an IP filter policy or an exceeded quota. Since policy rules haven't been modified, it must be a quota issue.
Note: Cursing your clients (in your mind) is a valid/invalid solution, depending on your morals.