The message
The error message needs to be clear, precise, short and punchy. Users should be able to immediately understand what ‘mistakes they have made’ and how to recover the error. This is fundamental and will have a huge impact if users can’t immediately understand what mistake they have made.
One example of an unclear error message is on the Hotmail registration page where it asks for user’s ‘Birth year’. I remember using only two digits to represent a year before the year 2000. Well, the form does not give any instruction on that; even the error message does not give a clear idea of what was wrong with entering two digits for my birth year.