Error LED.

Since we're seeing a lot of new boards that have many fewer user-accessible pins than there are IO pins on the microcontrollers being used, it would be nice to put an "error" LED somewhere on the board that system/core or user functions could light up when something goes wrong...

There is some discussion of a similar idea here:

I don't like the idea of blinking pin 13, as suggested in that issue, because that could often interfere with other use of that pin but I do like your idea of using an IO pin that's not otherwise broken out. This could be defined in a macro such as LED_ERROR so that no action is taken on boards that don't have an error LED available.

There's evidence that beginners instinctively expect something of this sort because I have seen quite a few posts here from people concerned that the red light blinking on their brand new board indicates a problem, when that's actually just the expected behavior of LED_BUILTIN on a new board.