Backend engineers often underestimate the long-term impact of early database design decisions. Starting with tables before understanding data relationships and usage patterns leads to slow queries, costly migrations, and structural debt. Key pitfalls include designing only for the present, ignoring foreign keys, premature denormalization, and treating the database as a simple storage layer. A better approach starts with domain entities and relationships, enforces constraints at the database level, uses appropriate data types from the start, and normalizes first before optimizing. The schema outlives most application code, so early decisions carry disproportionate weight.