This was uncovered during execution of the Core Functionality Tutorials, Chapter 2.

In the tutorial, an RPC call is made to a service provider before that code has been added.

The expected response is:

"error-type": "rpc",
"error-tag": "operation-not-supported",
"error-message": "No implementation for this operation is available."

When this call is made:

curl -H 'Content-Type: application/json' -X POST -d '{"input" : {"toaster:toasterDoneness" : "10","toaster:toasterToastType":"wheat-bread"}}' --verbose -u admin:admin http://localhost:8181/restconf/operations/toaster:make-toast

The response is:

"error-type":"application",
"error-tag":"operation-failed",
"error-message":"The operation encountered an unexpected error while executing."

This error message does not lead the developer to the cause of the error message.

Also, if there's a typo in the URL as follows:

curl -H 'Content-Type: application/json' -X POST -d '{"input" : {"toaster:toasterDoneness" : "10","toaster:toasterToastType":"wheat-bread"}}' --verbose -u admin:admin http://localhost:8181/restconf/operations/toaster:toaster

The error message is:

"error-type":"protocol",
"error-tag":"malformed-message",
"error-message":"Error parsing input: Schema node with name input wasn't found."

"malformed-message" is technically incorrect as the message is valid JSON syntax.