Error messages are the most important copy on any digital product. They appear at the moment when something has gone wrong — when the user is frustrated, confused, or worried — and they determine whether the user can recover and continue or gives up and leaves.
The three things an error message must do
A good error message tells the user what happened, why it happened (if useful), and what to do next. "Something went wrong" does none of these. "Your session has expired. Please sign in again to continue." does all three.
Specificity is kindness
"Please enter a valid email address" is more useful than "Invalid input." Specificity requires that error handling in the application be specific enough to produce specific messages. Generic error messages are often a symptom of generic error handling.
Tone in error messages
Error messages should be calm, clear, and non-accusatory. They should be matter-of-fact: here is what happened, here is what to do. The tone of an error message is a test of the brand voice under pressure.