Reading obsessively about Stripe for a month taught me one thing that changed how I build billing

Question

daily.dev handed me a "Top Reader" badge for the Stripe topic this month, which is mostly the platform politely telling me I have a problem. But the upside: it forced a pattern I now build around — every Stripe webhook handler has to be idempotent, because Stripe will deliver the same event twice and your "charge succeeded" logic will happily run twice if you let it. Dedup on the event ID before you do anything with side effects. It's the cheapest insurance in payments and the most common thing I see skipped.
What's the billing edge case that bit you hardest — duplicate webhooks, proration math, or failed-payment recovery?