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A list CRDT has one job — converge — so you can brute-force it: 400 operation logs, every one of the 720 delivery orders each, diff the results. Three orderings broke the naive implementation in three different ways, and one of them left every replica in perfect agreement about a scrambled document.

Eight concurrent workers sharing one idempotency key turned a $49 order into $392 of real charges on PostgreSQL 17.6, and adding a FOR UPDATE row lock changed nothing at all. This walks through what actually prevents the double charge, measured at every step, and what Stripe's and Shopify's own documentation says rather than what gets repeated about them.

If you're still writing pages/api/... files in a new Next.js project, stop. Route Handlers are the App Router's replacement, and they're meaningfully better — typed, streaming-capable, and built on the same Web platform APIs that Cloudflare.

Here's a scenario I see in almost every codebase I review. A developer writes an async function that reads some state, does a bit of work, then writes back a result. In tests, it's fine. In development, it's fine.