|Shared State Is Incorrect State|
You're in your favorite diner. You finish your main course, and ask your server if there's any apple pie left. He looks over his shoulder, sees one piece in the display case, and says yes. You order it and sigh contentedly.
Meanwhile, on the other side of the restaurant, another customer asks their server the same question. She also looks, confirms there's a piece, and that customer orders.
One of the customers is going to be disappointed.
Swap the display case for a joint bank account, and turn the waitstaff into point-of-sale devices. You and your partner both decide to buy a new phone at the same time, but there's only enough in the account for one. Someone—the bank, the store, or you—is going to be ...