[GrinningFool]

I would say it should be a big red alert box with a simple error code that the end user can reference when trying to find help.

The applications (CLIs, native, web, etc) I've seen that present me with non-actionable errors is a perpetual source of irritation.

"Failed to open file"

"File could not be uploaded"

etc

Not only are these useless to the user who can't do a thing about them except try the same thing again, they're useless to the developer or support engineer who might be trying to help them.

[sojournerc]

Related, the "oops, something went wrong" error messages absolutely infuriate me. Something about the tone, like a child spilling milk, and the uselessness of the message get under my skin.