clojure.core.async.impl.ioc-macros/return-chan registers an incorrect callback on a channel.
return-chan is registering a fn callback on a channel that takes no-args, callbacks on channels always take an arg. This results in an exception if the callback is ever called. I am not sure why, but the callback doesn't get called most of the time. However if you are (un)lucky sometimes you get a stacktrace like: