Error Code Help

Question should be in Products​, so if you could move it that would be great.

Salesforce API calls are not involved in this rate limit... your description of the suspected cause is hard to understand.

The rule is that integration(s) using the Marketo API cannot make more than 100 calls in 20 seconds. That's calls inbound to Marketo. 

One single-threaded app (meaning it makes only one network connection at a time, waiting for that one to complete before opening the next one) is unlikely to exceed 5 calls per second (not impossible, but unlikely).

However, a multi-threaded (or multi-process, etc.) app, or multiple apps, can overwhelm the limit if not specifically coded to stay within limits.  For example, an app that has 5 threads connecting to Marketo independently, without coordinating with each other, means you only need to complete 1 connection per second per thread to break the rules.

So I don't know what integrations you're writing/using, but a seemingly "robust" one may be the culprit, since aiming for higher performance without specific attention to rate limits can end up causing this error.  You also might look for apps that get launched by some kind of scheduler many times simultaneously (in this case, the individual app is innocent, but somebody decided to try to increase throughput by launching many times).

Bonnie Seaman​ Have you reviewed what connections you have making API calls? You can do this in Admin > Web Services > click on the number next to requests last 7 days. This might give you an indication of what your robust API connection is.