We shipped a job that sent order confirmation emails. It deduped by keeping message IDs in an in-memory Set. Clean code. An agent wrote it, I skimmed it, tests passed, staging was happy for two weeks.
Then traffic spiked, autoscaling added a second instance, and a few hundred customers got the same email twice. Two Sets. Zero shared state. Obvious in hindsight, invisible in review.
Here's the part that actually cost me: finding it took nine hours, and roughly seven of those were spent watching an AI agent confidently fix things that were not the bug. Vibe coding didn't hurt me. Vibe debugging did.
TL;DR
Vibe coding works because generation has a cheap oracle: you run it, you see if it does the thing. Wrong guesses die instantly.
