The issue seems to be with lib/proxy/ProxyServer and the connection pool (the Agent class) having a default setting of 5. I'm able to postpone the issue by setting the maxSockets to 10 instead of the default 5.

http.globalAgent.maxSockets = 10;

Now, the way I've been testing this is by refreshing the browser which causes a close a SockJS onclose event to be triggered. But this does not seem to "free" the current sockets to be returned to the pool. Adding a log statement that displays the an array of sockets that currently in use by the Agent shows that all five are in use when the issue occurs.

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