The phrase "Just connect it to our ERP" ruins eCommerce projects by treating complex brownfield integration like simple greenfield development. Legacy ERP systems are poorly documented, still running archaeological digs that require real-time coordination across unowned system handoffs. Payment authorization happens synchronously while downstream processes like order creation and stock reservation are asynchronous, creating a dangerous gap where customers see confirmation before systems can deliver.

When queued jobs fail silently, money is taken with no order behind it, forcing humans to manually reconcile data—a costly bottleneck. The fix requires upfront planning, explicit architectural patterns for eventual consistency, and budgeting integration as a first-class project, not an afterthought sprint task.