A lawn care company sells an Essential package and a Premium package. They're mutually exclusive — you get one or the other, never both. But the product catalog didn't know that. A customer on the widget could select both, see a combined price that makes no sense, and submit a quote that the office would have to manually correct.
An excludes_products JSONB column on the products table now stores an array of product IDs that become unavailable when this product is selected. The tenant configures it through a "Conditional Display" panel in the product editor — a MultiSelect that lets them pick which products conflict. Select Essential and Premium goes gray with a lock icon and a tooltip: "Not available with Essential." Auto-deselect handles the reverse — if Premium was already in the cart when Essential gets added, Premium drops out.
No junction table. No complex relationship management. A JSON array on the product row, a computed property on the store that collects all excluded IDs from the current selection, and a disabled state on the product card. The simplest enforcement of a business rule that existed in the operator's head but not in the software.