No `throw new Error("not implemented")` in production code

A throw new Error('not implemented') turns into an HTTP 500 the first time any caller hits that code path. Because AI generates these as scaffolding — calculateRefund, sendInvoice, getUserBalance — the thrown paths often sit directly on revenue-critical flows. Users encounter them during checkout, refund, or billing and abandon the transaction; the defect then masquerades as a generic 500 in observability and takes hours to trace back to the stub.

Implement the function against real logic or remove it and adjust every caller. Do not leave scaffold throws behind; a // TODO next to a thrown error is still a 500 in production. Fix at src/lib/billing.ts:

export function calculateRefund(amount: number) {
  const feePercent = 0.03
  return amount * (1 - feePercent)
}

• ID: ai-slop-half-finished.incomplete-impl.throw-not-implemented

• Severity: medium

• What to look for: Walk all non-test source files. Count all throw statements whose error message matches this regex (case-insensitive): \b(not\s*implemented|not\s*yet\s*implemented|todo|implement\s*me|coming\s*soon|placeholder|stub|fixme|unimplemented|notimpl)\b. Also count throw new NotImplementedError() regardless of message. Also count throw 'not implemented' and similar string throws. EXCLUDE abstract class method bodies (files containing abstract class) AND interface stubs.

• Pass criteria: 0 throw statements with "not implemented" messages in production code. Report: "Scanned X source files, 0 'not implemented' throws."

• Fail criteria: At least 1 source file contains a throw with a "not implemented" style message.

• Skip (N/A) when: Project has 0 source files.

• Detail on fail: "2 'not implemented' throws: throw new Error('Not yet implemented') in src/lib/billing.ts line 34 (function 'calculateRefund'), throw new Error('TODO: implement') in src/lib/email.ts line 12"

• Remediation: A throw new Error("not implemented") means any code path that reaches this line will 500. AI often generates these as "scaffolding for later" — but the AI never comes back to replace them. Either implement the function or remove it and fix the caller:

  // Bad: function stub that throws
  export function calculateRefund(amount: number) {
    throw new Error('Not yet implemented')
  }
  
  // Good: implement it
  export function calculateRefund(amount: number) {
    const feePercent = 0.03
    return amount * (1 - feePercent)
  }