Many startups generate disproportionate revenue from a small number of customers, creating existential risk if those customers leave or reduce spending. This episode examines why founders rationalize customer concentration as temporary, assuming they will diversify as the business grows, when in reality concentration often persists or worsens. We explore the specific scenarios where customer concentration creates vulnerability: enterprise software companies dependent on a few large contracts, SaaS companies where a single customer represents 20 percent of revenue, and marketplaces where a small number of sellers generate most volume. The episode analyzes how customer concentration affects valuation, investor perception, and business sustainability, and why sophisticated investors view concentration as a major red flag. We also examine the operational and psychological factors that perpetuate concentration: founders overserving large customers to maintain relationships, founders avoiding aggressive sales to smaller customers because large customers are easier, and founders' fear of losing major customers if they attempt to diversify.