I used to work for a company that had a main website and a lightweight app. LW app was distributed to partners and added to other sites using an iframe.

Someone decided a requirement was to retain the shopping cart for anonymous users. Some dev thought the best way to do that was to issue auth cookies to anonymous users.

The auth cookie issued by the LW app was actually for the main site. A few users for LW app decided to just come to main site to make a purchase. Since they already had an auth cookie (issued from LW app), they were never prompted to log in, create an account, or use guest checkout on the main site. They were still able to complete their order and we had their shipping address, but we didn’t have their email address so we couldn’t contact them about their order.

Customer service had no way to email customers if something went out of stock or if there was a product recall. CS would have to call these customers and ask for email addresses. Good luck getting anyone to answer or return a call nowadays. Customers were asking where their confirmation email was. The admin website was polluted with “users” that had the placeholder email for non-logged in users.

This happened because of a combination of an understaffed and overextended engineering department. Of course when something goes bad it’s going to be bad.