From @Apollon77 on September 20, 2015 20:53

Copied from original issue: arduino/Arduino#3829

I use a library where the EthernetClient instance is created once and then reused.
This was not really working - the first call worked, then it did not connect for additional uses.

I tracked it down to the following:
The library did the following:

if (udata->client->theClient->connected())
{
  udata->client->theClient->stop();
}

And this was the reason! After all data was received the called server closed the connection. So the socket was in status CLOSE_WAIT and available returned 0 because all data was already read. That's why stop was never called.
And that's why the connect method found a still set _sock to a valid number and returned false.

I found the advice to use "if connected() then stop()" in several sources in the internet, but this brings big problems as seen here.

I think the fix should be to change the check in connected: All Sockets that are in CLOSE_WAIT are reused in any case ... so a call to connect should check also connected and call stop or simply reuse that socket also when _sock is a value < MAX_SOCK_NUM.