Ask HN: Any examples of shadow-testing in production environments?

Lately I've been curious about shadow-testing code in production, by which I mean if I have a v1 build in production, I'm interested in capturing and replaying api calls on a v2 build in production on a box taken out of rotation from being discovered by load balancers. This could be enabled by having a proxy that "re-dispatches" the request to the v2 build after getting the v1 build response and essentially diffing the results. One of the limitations that comes to mind is that this may not work for non-idempotent api calls (ex: making a payment) but for the rest, it seems like a valuable addition to a CI pipeline to effectively test your code in production in a safe controlled way.

Have any HNers here use such an approach in their work? Any words of advice/caution that I should be aware of before investing some time into building something like this out?