Clients hitting old pool servers after pool change

Out of curiosity I tested on a lab environment and in my case the Persistence did not send the traffic to a server that's no longer in the Pool. This is on v13 so it might be a different behavior from your version though.

One thing that just might be a case though is if it was existing connections in the connection table, because those aren't deleted even when you delete the objects involved. In fact you can delete the VS and the BIG-IP will still forward traffic that matches those connections. In my experience HTTP connections doesn't stay open for very long but if there's data going all the time I guess it is possible.