Obviously you want to ensure the Debug Console is turned on. Once done, if you return to the browser and rerun your page with the error, you will see a new notice:
Clicking that option will give you some details about the error:
It's no Firebug, but it's better than nothing.
So what's the second tip? The error the user had involved a simple jQuery call to a CFC that returned JSON. It worked perfectly everywhere but the iPhone. Why? The iPhone didn't like the fact that the result didn't have the proper mime type. I asked him to add this line before his cfreturn: <cfcontent type="application/json" reset="true">. Unfortunately this kinda breaks the reusability of the component. To be a bit more abstract, he could add a quick check for URL.returnFormat first and only use the content type when necessary.