Only occurred once during testing, was unable to reproduce again, might involve inotify event ordering? This likely happened during pipe refresh after deleting an endpoint. This is most likely a timing issue (I think).
HOPPER /run/user/1000/hopper
CREATE test(1)
CREATE test/test2(2)
OPEN +test1(test)
UP +test1(test)
OPEN -test2(test/test2)
DOWN +test1(test)
CLOSE +test1(test)
Exception: open: No such file or directory: No such file or directory