d1cf9e77_erience_in_ArnoLand__July_2009__Yes_but___
[Heuristic] Help users recognize, diagnose, and recover from errors [Justification (From Nielsen)] Error messages should be expressed in plain language (no codes), precisely indicate the problem, and constructively suggest a solution. [Yes but...] 
My only problem here is the "precisely indicate the problem." I am sure Jakob did not mean go into gory technical details of the problem, but rather concisely describe the issue. E.g. "Your data was not saved." not "Your data was sent to the application layer and experienced a time out longer than 3 ms and the system sent back the data in an unusable format." My formula for error message writing: "Short sentence what happened. (forget why) Short sentence how to fix it. A link can be added "Learn more" or "Why did this happen to such an undeserving user as me" for the morbidly curious.
User Experience in ArnoLand: July 2009
Yes but...