I am working on a tiny hardware device which has a single LED that can be used to display errors (there are no other outputs). Actually it is a tri-color LED but the red color is dedicated to errors. Hopefully that is user-friendly.
The question is: there are many possible kinds of errors, how to indicate what kind has occurred, and what to do about it? Ideally this should be obvious without reading the manual, based on similarity to other small gadgets/cellphones/etc. At least it should be memorable enough that it would only require flipping through the manual briefly once.
The most common error is that the battery is almost-dead and the device needs to be recharged before being used. This error would be communicated whenever you try to use the device.
Another common error is that the device is not connected correctly to the things it needs to be connected to, and it needs to be re-plugged-in.
There are other (rare) errors which should prevent the device from being used without factory service. It should be clear to the user that this is a permanent/non-user-fixable situation, and they need to call support; and maybe indicate the detailed kind of error in this case (let's say there are ~15-20 possible kinds) in a way that is readily described over the phone so that we can get some information about failures from support calls.
Note: the device has a tri-color LED (all colors independently dimmable and blinkable) and three buttons; that's it as far as available UI :)