Since I don't know why the problem occurs, I will give a real example of what happens. Basically what happens is that a terminal device used by screen in a previous session doesn't get closed sometimes after terminating screen and logging out. % uname -sr NetBSD 3.0.0_STABLE % screen --version Screen version 4.00.02 (FAU) 5-Dec-03 Example: I login as "user1" and notice that a previous screen session's tty is still open. Notice that the date is 3 days ago. % who user1 ttyp0 Oct 27 08:29 (123.123.123.123) user1 ttyp3 Oct 24 06:15 (:ttyp0:S.0) hows:netb% screen -list No Sockets found in /home0/nbsd/.screen. Then I login as another user (user2), and check what that user sees. % who user1 ttyp0 Oct 27 08:29 (123.123.123.123) user2 ttyp1 Oct 27 08:31 (123.123.123.123) user1 ttyp3 Oct 24 06:15 (:ttyp0:S.0) Then I login as yet another user (user3), and also start screen by the first user. Now ttyp3 is grabbed by that third user, and the 1st user's screen is now on ttyp6 and updated. % who user1 ttyp0 Oct 27 08:29 (123.123.123.123) user2 ttyp1 Oct 27 08:31 (123.123.123.123) user3 ttyp3 Oct 27 08:35 (123.123.123.123) user1 ttyp6 Oct 27 08:54 (:ttyp0:S.0) If I start screen by user2, then I no longer see user1's screen entry, only user2's. % who user1 ttyp0 Oct 27 08:29 (123.123.123.123) user2 ttyp1 Oct 27 08:31 (123.123.123.123) user3 ttyp3 Oct 27 08:35 (123.123.123.123) user2 ttypa Oct 27 09:19 (:ttyp0:S.0) User3, already in screen, sees the same thing (not his own screen tty): % who user1 ttyp0 Oct 27 08:29 (123.123.123.123) user2 ttyp1 Oct 27 08:31 (123.123.123.123) user3 ttyp3 Oct 27 08:35 (123.123.123.123) user2 ttypa Oct 27 09:19 (:ttyp0:S.0) Then user3 terminates screen and sees: % who user1 ttyp0 Oct 27 08:29 (123.123.123.123) user2 ttyp1 Oct 27 08:31 (123.123.123.123) user3 ttyp3 Oct 27 08:35 (123.123.123.123) Then user3 starts screen again and sees his own screen's tty: % who user1 ttyp0 Oct 27 08:29 (123.123.123.123) user2 ttyp1 Oct 27 08:31 (123.123.123.123) user3 ttyp3 Oct 27 08:35 (123.123.123.123) user3 ttypb Oct 27 09:23 (:ttyp3:S.0) So what is user1 seeing now that screen has started: % who user1 ttyp0 Oct 27 08:29 (123.123.123.123) user2 ttyp1 Oct 27 08:31 (123.123.123.123) user3 ttyp3 Oct 27 08:35 (123.123.123.123) user1 ttyp5 Oct 27 09:29 (:ttyp0:S.3) But user2, after terminating screen sees user1's screen tty at ttyp6, not ttyp5: user1 ttyp6 Oct 27 08:54 (:ttyp0:S.0) Then if user3 starts screen, user2 sees yet another picture, not user1, but now user3's screen terminal: % who user1 ttyp0 Oct 27 08:29 (123.123.123.123) user2 ttyp1 Oct 27 08:31 (123.123.123.123) user3 ttyp3 Oct 27 08:35 (123.123.123.123) user3 ttyp9 Oct 27 09:34 (:ttyp0:S.0) Strangely, user1, still in screen, now sees exactly what user2 sees % who user1 ttyp0 Oct 27 08:29 (123.123.123.123) user2 ttyp1 Oct 27 08:31 (123.123.123.123) user3 ttyp3 Oct 27 08:35 (123.123.123.123) user3 ttyp9 Oct 27 09:34 (:ttyp0:S.0) Now user2 and user3 both terminate screen and logout, and user1 terminates screen. There is still a screen tty open, but now at ttyp6, not the starting ttyp3. So the "orphaned" ttyp only changed position and was updated, but still screen cannot get rid of it. % who user1 ttyp0 Oct 27 08:29 (123.123.123.123) user1 ttyp6 Oct 27 08:54 (:ttyp0:S.0) So everyone logs out, even user1. Then user1 logs in again, but that ttyp6 is still open: % who user1 ttyp0 Oct 27 09:55 (123.123.123.123) user1 ttyp6 Oct 27 08:54 (:ttyp0:S.0) If user1 starts screen then ttyp6 disappears, and the screen ttyp is ttyp3: % who user1 ttyp0 Oct 27 09:55 (123.123.123.123) user1 ttyp3 Oct 27 09:57 (:ttyp0:S.0) But if user1 terminates screen, only the ttyp3 goes away. The "orphaned" ttyp6 is still open. Usually, the only way to get rid of an "orphaned" ttyp is to reboot the machine. However, if nothing is done about the situation for a few weeks, sometimes it mysteriously goes away.