Both the EC2 instance health and our HTTP health checks. If either of those failed the server would have been removed from the load balancer, but they didn't fail.

Only the external health checks that hit the system from an outside service were failing. And because those spread out the load across the AZs, only a fraction of them were failing and no good way to tell the pattern of failure.

I did have some Kubernetes pods become unhealthy but only because they relied on making calls to servers that were in a different AZ.