My Shopify checkout conversion rate suddenly dropped. What should I check first?

Once, during a weekend promotion on a DTC independent site, the problem didn't seem big at first: traffic and add-to-carts were normal, but the checkout conversion rate suddenly dropped. The ad team thought the creative was the issue. Everyone's explanation on-site sounded reasonable, but they didn't add up when put together. I first had everyone pause to prevent further impact, then checked the…