Continuing on my list of programming dictums that I invented over the years.
Error messages are an indication of failure on the part of the programer, not of the user.
If a user isn't supposed to do something, then the program shouldn't let him or her do it. Classic example: If you want the user to type a number, throw away any keystroke that isn't 0-9 or editing keys. Do not allow the user to type in "monday" and then throw an error message on the screen.
What's more, an error message should be short, but contain enough information for a programmer to reproduce them. In my large application at work, I save pretty much every variable I can get my hands on to a log file. I then display the log files name to the user. Users nearly always send me a screen cap of any error message they encounter (embeded in a .doc, but that's another story) so I can get the ID from there. Of course, I can also just look in the log directory for new error logs. Taking this one step further, the app could email me the ID when an error occurs.
