Well-designed systems hide their internal complexity until you violate an assumption. A service might work flawlessly for months, then suddenly fail when you pass it data that’s technically valid but crosses an undocumented boundary (wrong timezone, unexpected null, edge-case enum value).
The debugging pattern: When a stable system suddenly breaks on “valid” input, you’ve discovered an implicit contract. The fix isn’t just handling the edge case—it’s making the boundary explicit through validation, type constraints, or documentation.
The architectural lesson: Every system has assumptions about its inputs, state, and environment. If those assumptions aren’t codified (through schemas, contracts, or explicit validation), they become landmines. The best architectures make their boundaries impossible to cross incorrectly, not just expensive to cross incorrectly.