When a Rate Limit is configured on an API Plan or Account Plan in the Portal, it will cause API calls to be throttled to stay underneath the value set for the Rate Limit.
By default, the Portal will configure the Rate Limit assertion (found in the associated policy viewable in Policy Manager) with the value chosen earlier for the Maximum Requests Per Second function, and have the Spread Limit Over X Sec Window function disabled, and an action set to Throttle. This does not normally cause issues with typical traffic volumes, but depending on the frequency of traffic/API calls (especially if it's being load tested), some requests can be throttled in order to stay under the value set for the Maximum Requests Per Second function, negatively impacting the API client sending the request.
The Rate Limit Assertion does some math for the Maximum Requests Per Second, which is not always well understood. If the value is set to 100 (for example), then the Rate Limit assertion will process all traffic as expected as long as there is no more than 1 request per 1/100th of a second. If two calls come in at under 1/100th of a second apart (which often happens when using load testing tools) for example, then the second request will be throttled. If a request is throttled, it is considered a failure case and will throw the error to the client.