The design fix depends on the spacing the product needs. If control events are infrequent with guaranteed spacing, a toggle synchroniser with a stated spacing rule closes the loss at the cost of two slow cycles of latency. If events can arrive back-to-back or software can re-trigger quickly, only a handshake or FIFO with acknowledgement is correct, at the cost of throughput and logic. Either way the two-flop-only path is removed, and the choice is recorded with its assumption so the next engineer cannot reuse the toggle where spacing is unguaranteed.
The patch changes the protocol, so every proof is re-run: structural analysis on the changed RTL, the liveness assertion that every source event yields exactly one destination event, and metastability-aware simulation of the new handshake under traffic. The trap from the scenario is skipping this step because the patch "obviously" works. Nothing about a crossing is obvious at the corner; the re-proof is the fix, the RTL edit is just its subject.
Escapes recur when reviews accept unnamed synchronisers. The durable change is a review rule: every pulse crossing names its scheme and its spacing evidence before signoff, and any "two flops" on a pulse path is a review defect, not a style comment. Add the width analysis to the standard checklist alongside the structural report, so the gap found here becomes a gate every design passes. This is the Trade-off step applied to process: one checklist row against every future bring-up week.
Because the proof covers the implemented protocol under all alignments, and the patch replaced the protocol. A textbook scheme wired into new surrounding logic is a new claim needing a new proof.