When I tried to register to post a usability issue in WordPress I promptly stumbled about this one concerning the forums:
Problem: When registering for the forum, one gets a misleading and less than helpful error message
Detailed Description: I gave "pallas" for a user name and after sending the form got the following error message: "Your username was not valid, please try again"
The name is not invalid, it is just already taken. Why doesn't the message say so??
Maybe to your system that makes a name technically invalid, but as a user one is mislead to think that something is wrong with one of the letters.
There really is something wrong with one (or more) of the letters. Why don't you tell the user which one (umlauts, accents, ...). Without this information you let your user poke around in the dark.
- For Possibility 1): Change the text to: "Unfortunately your user name was already taken. Please pick a different one."
-- Even better: Check the user name availability in real time with AJAX, to save the user the trouble of sending the form when there is no chance of succeeding.
- For really invalid names (Possibility 2):
1) Check with an open mind, whether there is a need for some characters to be invalid, or if you just don't want to go into the hell that is encoding.
2) If some characters have to be invalid at least tell the user which those are!