The Node interface started returning 409 after going live today. How do I troubleshoot compatibility issues?

This morning, I was rolling out a minor version update for a long-term client. As soon as the frontend switched to the new interface, it started intermittently returning 409s, which I couldn't reproduce in the test environment. At first, I almost wanted to roll back, but then I matched the access logs, request IDs, and database unique index errors. I discovered that the old mobile version was…