What breaks on a store during Black Friday?

Stock counts, checkout under load, and the support queue. In that order and usually all three, because each failure feeds the next.

In more detail

The compounding is the danger: overselling produces cancellations, cancellations produce tickets, and the queue is longest exactly when answering quickly matters most. Reserve support capacity as deliberately as you reserve stock.

What to do about it

Discounting into a crowd where everybody is discounting wins volume and loses margin. The stores that do well have a reason to be chosen that is not the price.

Traffic spikes break the things that were nearly broken already — usually image weight and a checkout step nobody tested under load.

The customers won in that week are worth almost nothing unless something brings them back in January.

The mobile app angle

This is the one week where owning a notification channel pays for the year — reaching last year’s buyers costs nothing per message.