• If a user makes a mistake i.e. an error occurs, they should get feedback on screen informing them of this.
  • A good example of this is saying in an email that you'll attach a file but don't.
  • Gmail now reminds you of this before you hit send.
• The error message should visually stand out on the screen - ensure contrast
  • Try to use a different color to the rest of your color palette.
• Never just rely on color to convey a message to ensure accessibility for people with color-blindness.
  • Use icons and other UI elements in addition to color.

• Don't use the default code output as an error message as this will be very hard to decipher for the user.
• This type of detail belongs in the logs.

• Don't use terminology that blames the user for making a mistake
• Avoid:
  • “you did..”
  • “your action caused..”
  • “you have entered the incorrect information”
  • upper case and ! at the end of a sentence - it seems like you're shouting at the user!
• Blaming the user is only going to cause more frustration
  • Don't see the user as the problem.
  • We cannot know how every type of user will interact with what we design so errors are bound to occur.

• You need to tell the user:
  • What happened
  • Why it happened
  • what the user can do to resolve the issue
• Guide the user on how they can fix the error.
• If possible, determine what the correction action is a and let users pick from a small list.
• Users should be able to understand what error message buttons (delete, cancel) do by simply reading their labels.
  • You shouldn't need to explain what the buttons do

• Most users will scan a page to find exactly what they're looking for rather than reading everything individually.
  • As a result, the more text on screen, the harder it is to scan, and the less likely it is for the user to read the error message.
  • Remove any unnecessary detail.
• Use progressive disclosure to provide information on how to troubleshoot a problem if it's required.
  • Don't put all of this in the error message box.

• When a user is interrupted to deal with an error, it stops their flow and prevents them from completing their task.
  • Make it easy to resolve the issue so they can go back to their work.
• Let users correct where they went wrong rather than starting from scratch each time

When designing an error message ask yourself:

Does the user need to be interrupted to read this message?

If so consider putting the error in a modal.

If not, don't use a modal.