Based on the patterns we observe in fintech MVP projects, the mistakes made by fintech startups that cause the most downstream damage are: treating compliance as a post-launch concern, which creates blockers with banking partners and payment processors; skipping product validation before building, which can lead to significant engineering investment in a product that has not confirmed market demand; and building tightly coupled architectures that become costly to extend. Security gaps and insufficient transaction testing also appear frequently and are worth addressing explicitly in the MVP scope.