One of our clients had a problem that when they try to delete users who have signed up via mobile app they get an error saying they must enter a street address for the user before the account can be deleted. This was because users who sign up via the mobile app were not asked for their street address, but on the Drupal site the street address is a required field when a user signs up.
Profile Validation
Lets consider a situation in which you are using addthis widget for sharing a page in your site via email and you want to attach an image with the mails that is been send from your site. In the article you can find out how to add a common image to the all mails send from a site and also how to attach images depending on different users.