Here is a bug report I have received in some form at three different companies. The customer opens the payment widget, enters their card, and hits pay. While the bank is doing its 3-D Secure dance, the cart's countdown timer hits zero. The frontend, doing exactly what it was told, tears down the session. The payment succeeds a second later against a cart that no longer exists. Now there is money in the account and no order attached to it.
Every engineer who built that timer built it correctly. The countdown protects a limited resource: a concert seat, a hotel night, the last unit in stock. Holding it forever would let one abandoned tab starve every other customer. So you set a timer, and when it fires you release the hold. Clean, defensible, and completely blind to the fact that a real person had already committed their money.
That blindness is the actual subject here. The countdown-expiry bug is small. The habit that produces it is not.
The timer answers a question nobody asked at that moment
Step back and ask what the countdown is for. It exists to resolve contention: two people want the same seat, and the hold decides who gets first refusal. That is the only job. It is a fairness mechanism between customers who have not yet paid.
