If your account has reached its API limit, you will receive the error message "API Rate Limit Exceeded" and you will be unable to authenticate with the DataSift API.
Each REST API call made has a cost associated. Exceeding your limit will temporarily suspend any additional calls until the limit clears (up to an hour). More information can be found here: http://dev.datasift.com/docs/rest-api/api-rate-limiting
Note, this limit does not affect the flow of data. It will prevent calls to things like compiling new CSDL, checking balance/usage, starting/stopping Push subscriptions, etc.
It is possible to monitor the current API consumption through the headers from each REST API call (the "X-RateLimit-Remaining" value) for debugging purposes