Browsers are so smart they are trying to fill forms for you. And sometimes this is bad.
Html5 provides an attribute autocomplete="on|off" to prefent browsers filling fields for you. And you should add autocomplete="off" to inputs in cases like these:
- "Set password", "Change password" and "Retype password" fields.
- Captcha field.
- Credit card number.
Sometimes you need to do it for security reasons. Sometimes it just makes your forms more comfortable.