Investing and trading app testing

What breaks in trading apps and what it costs

Trading apps earn inside each order and each financed position, so one defect in routing, margin, or entitlement code repeats across every affected account before a statement run exposes it.

• Payment for order flow booked against the wrong order. A cancel-and-replace gives the order a new ID, and the rebate allocation job matches the wholesaler's statement only against original IDs. Payments on amended orders fall into an unallocated bucket, so the payment for order flow revenue line and the per-venue report disagree. SEC Rule 606(a)(1) requires a quarterly public report of the net payment for order flow received from each venue, as a total and per share, published within one month after the quarter ends under 606(a)(2). A misallocated rebate makes that public report wrong for each venue it touches.
• FX spread left out of the annual cost report. An EU app charges a currency conversion spread on US-listed shares and books it in a separate revenue ledger that the costs-and-charges feed never reads. MiFID II Article 24(4) requires aggregated costs and charges, where applicable at least annually, and Annex II of Delegated Regulation (EU) 2017/565 lists foreign exchange costs and mark-ups embedded in the price as transaction costs. Every client with USD trades receives a statement that understates total costs, and the correction is a reissued statement run.
• Tiered margin rate applied to the whole debit balance. The disclosure charges each portion of the balance at its own tier rate, but the engine applies the first tier's rate to the full balance. On a synthetic 50,000.00 debit with 8.00% up to 25,000.00 and 6.50% above, on a 360-day year, the overcharge is about 1.04 per day. SEC Rule 10b-16 requires disclosure of the method of computing interest and periodic statements with the interest charged, so every statement above the tier boundary contradicts the disclosed method and becomes a refund. Our interest accrual and day-count guide shows the boundary cases.
• Maintenance check on a stale price. The market data handler reconnects after a gap, and the risk engine keeps the last price without a staleness flag. During a fast drop, account equity stays above the house requirement and the 25% minimum in FINRA Rule 4210(c), so no margin call or liquidation starts. When fresh prices arrive, account equity is negative and part of the margin loan is unsecured. Under SEC Rule 15c3-1(c)(2)(iv)(B), deficits in customers' unsecured and partly secured accounts are deducted from net worth when the broker-dealer computes net capital.
• Ex-dividend date one business day early. US equities have settled on T+1 since 28 May 2024 (SEC), and FINRA Rule 11140(b)(1) now sets the ex-dividend date on the record date for distributions under 25% of the share price. An entitlement engine that still uses the T+2 offset credits the dividend to customers who sold on the business day before the record date. The broker pays those sellers cash the custodian never paid for their shares, reverses both sides, and recalculates margin interest on every debit balance the cash changed.