A guide to exception messaging

I am a huge believer in making systems just work. I have never been more proud of my design than when it gets all the way to production without a help system of any sort. There's just nothing to say! Still, there are almost always error messages, of one sort or another. Ideally, these are assistive, and actually preempt errors. I am tending lately to call these "exception messages." Some good ones have been messages shortly before Christmas on e-commerce sites stating whether products can be expected to get there in time (sorry, I forgot to take a screenshot) and some bad ones have been inappropriate use of orange road signs. Like the street signs, its sometimes helpful to look to machine-world examples of such things. Its winter here, and for some reason a lot of places like to lock one of their two entry doors, all winter. I think they believe that they think it cuts down on the draft, but its pretty dumb to me. Pretend, though, that its just broken. If you wanted one door forever, the right answer would be to board it up; that's a way to avoid anyone trying to use it, so no errors occur. But if its temporary, how do you message it? Well, not like this: This is actually one of the better signs I have seen on doors in my neighborhood. (Also note its at the front of an airlock. Any draft is already caught by the second set of doors, so its extra inexplicable). Its not very clear, so I end up tugging on lots of locked doors, which just a few weeks before I had been able to use. On the other hand, take Taco Bell. Our local one (I don't go there, but its right at the end of the street) has been abandoned, and its moved across the street. But you know how fast food joints have an iconic nature; they look like that same place years later. Old Taco Bells still look like you can get tacos, even if they serve Chinese, or are pawn shops. What to do? How about that? VERY large signs, make sure they are places that matter (e.g. the drive thru menu which is already attracting attention, and an expectation of reading), leave the lights on so the signs can be read, and make the message very, very clear. Its not /exactly/ across the street, but its so close you literally cannot miss it if you pull back out and look. No address, no diagrams, no arrows (typical for moved businesses). "Across the street." I find this to be a wonderful solution. How else can exception messages matter? How's life and death? This A310 crash is, partly, the result of not having any useful warning of disengagement of a part of the autopilot. A small light, and unusual operation compared to older aircraft (you can manually control only the roll channel but everything else is on autopilot still) is all that notifies. As this is considered a feature (not a bug) by Airbus, it makes some sense that there are not horns or anything, but perhaps something more clear would have been helpful. When things go really bad, certain elements simply disappear from the MFDs. There's still no actual message that something is wrong, much less /what/ is wrong. The solution, of course, was to just add more procedures and training.