See this thread for reference: https://groups.google.com/forum/#!topic/clojure/UfTQvYNf1-s

When a session is left idle for a little bit of time, the thread id evaluating forms changes. This causes any thread local variables in use to reset to their initial values. This causes strange behavior when doing interactive programming that involves use of thread locals.

The solution is for the same thread to always be used for a given session for as long as the session is open.