I always think that putting an exclamation mark at the end of an error message is the equivalent of adding "you idiot!". "Value must be numeric, you idiot!" "Date must be in d-ddd:mmm/yyyyy format, you idiot!" "Credit card number must not include spaces, you idiot!" (world's most annoying message: why can't you remove the spaces yourself?) Or in "success" messages: "File has been saved!" (like it's some sort of special achievement, and the computer is being extra kind to you in doing it)