I don't see in https://html.spec.whatwg.org/multipage/canvas.html#dom-canvas-transfercontroltooffscreen where we throw an exception if the canvas "has transferred its control to offscreen via calling {{domxref("HTMLCanvasElement.transferControlToOffscreen()")}}", can you point to it?

The transferControlToOffscreen() method, when invoked, must run these steps:
If this canvas element's context mode is not set to none, throw an "InvalidStateError" DOMException.
Let offscreenCanvas be a new OffscreenCanvas object with its width and height equal to the values of the width and height content attributes of this canvas element.
Set the placeholder canvas element of offscreenCanvas to a weak reference to this canvas element.
Set this canvas element's context mode to placeholder.
Return offscreenCanvas.The transferControlToOffscreen() method, when invoked, must run these steps:
If this canvas element's context mode is not set to none, throw an "InvalidStateError" DOMException.
Let offscreenCanvas be a new OffscreenCanvas object with its width and height equal to the values of the width and height content attributes of this canvas element.
Set the placeholder canvas element of offscreenCanvas to a weak reference to this canvas element.
Set this canvas element's context mode to placeholder.
Return offscreenCanvas.

After calling transferControlToOffscreen(), the canvas's context mode will be set to "placeholder"; so if calling transferControlToOffscreen() again, it will fail at the first step.

Sample code is as follows:

Thank you! (I didn't doubt you were right here, I just couldn't see it!)