Error prevention

Preventing an error is better than explaining it afterwards. In the "Easy to miss" form, the end date is before the start date, but nothing warns the person.

In the other form, the field is highlighted, a message says "Choose a date after 18 June" and Continue is disabled until the problem is fixed.

Errors come in two kinds:

• Slips: people mean to do the right thing but do it wrong, such as a typo. Prevent them with constraints, suggestions and forgiving formats.
• Mistakes: people misunderstand how something works. Prevent them with clear labels and good defaults.