Why Would a Loadbalancer Send Traffic to an Unhealthy Target?

Joe asked:

I have several application loadbalancers in AWS and came across this "feature":

"If there is at least one healthy target in a target group, the load balancer routes requests only to the healthy targets. If a target group contains only unhealthy targets, the load balancer routes requests to the unhealthy targets."

This is contrary to other AWS ELB documentation stating that unhealthy targets get removed from the traffic pool, and is contrary to my understanding of loadbalancer functionality, so my question is what could the possible rationale for this be? Why would it be to my benefit of any loadbalancer routing traffic to an unhealthy target under any circumstance.

My answer:

As the documentation says, this only happens if all targets are unhealthy. In this case it’s much more likely that your targets are up but your health check is broken, and routing to "unhealthy" targets means your site doesn’t experience needless downtime.

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