Electronic banking portals often sit in front of enterprise resource planning (ERP) systems such as SAP, mediating payment requests between users and back end financial infrastructure. When these integrations place excessive trust in client supplied HTTP metadata, subtle design flaws can arise that undermine payment integrity. This article presents a retrospective, anonymized case study of an SAP based payment flow in which weaknesses in HTTP level validation allowed the front end application to incorrectly treat unpaid transactions as completed. Rather than provide a reproducible exploit, we abstract the scenario into a general vulnerability pattern, analyze contributing architectural decisions, and propose concrete design and verification practices for secure web to ERP payment processing. The discussion emphasizes formalizing payment state machines, strengthening trust boundaries, and incorporating regular security review into integration projects.