Nine thousand doormats

The mistake

The promotion rule awards a free item for every qualifying item in the basket, and then re-runs over the basket contents including the items it has just added for free. Because each generated free doormat is itself treated as a qualifying line, the rule feeds its own output back into itself and multiplies with every pass instead of terminating. With no cap on total awarded units or on recursion depth, a single paid doormat expands until some other limit stops it — and the resulting order is dispatched in full.