Why do I get the error Failed to load resource?

Is there a general reason why I would get the error “Failed to load resource”?

What are the next steps to debug?

In my specific situation, I am getting the error followed by the service URL when trying to invoke a custom apex action.

Error:

Failed to load resource: the server responded with a status of 400 (Bad Request)
Service URL:

https://skuid.XYZ.visual.force.com/services/data/vXY.Z/actions/custom/apex/InvocableClassName