Based on the 7.1 Retail SDK, but likely applicable to later releases unless this has since been fixed.
A customer reported several transactions as "missing," backed up with CCTV footage and a credit card slip. Digging into the server logs, the same transaction ID showed up across two separate transactions — the cart wasn't being cleared between them.
My customization needed to access cart information right after checkout, so the cart-clearing logic lived in PostEndTransactionTrigger.
Root cause
A support ticket with Microsoft turned up the actual cause: if the cashier exits MPOS without closing the Change Due dialog, PostEndTransactionTrigger doesn't execute at all.
Takeaway
If you rely on PostEndTransactionTrigger, keep the logic inside it non-critical. It may silently not run if the cashier exits MPOS before dismissing the Change Due dialog — as ours did, with a stale cart as the result.