The method that finds all of these: map the workflow, list the assumptions each step makes about price / quantity / coupon / order / uniqueness / rate, then break each one. Client validation is UX; the server is the only control that counts.
⬜ Price manipulation: server trusted a client-supplied item price
⬜ Negative quantity: a negative line total credited the wallet
⬜ Fractional quantity: 0.5 qty bought at a rounded-down price
⬜ Huge quantity: an integer-overflow-ish total wrapped tiny/negative
⬜ Coupon reuse: a single-use coupon was applied more than once
⬜ Coupon stacking: multiple non-stackable discounts were layered
⬜ Workflow bypass: order confirmed without the payment step
⬜ Parameter tampering: isPaid/status/role flipped to get goods/privilege
⬜ Replay / idempotency: the same refund request credited more than once
⬜ Race / double-spend: a single-use code was redeemed twice
⬜ Captcha bypass: the server accepted a POST with no valid captcha
⬜ Reward loop: a self-referral farmed the loyalty bonus repeatedly
⬜ No rate limit: an OTP endpoint accepted unlimited attempts