diff --git a/man2/fcntl.2 b/man2/fcntl.2
index a68a3821b..daa3981dc 100644
--- a/man2/fcntl.2
+++ b/man2/fcntl.2
@@ -931,6 +931,19 @@ with
 .B O_NONBLOCK
 set etc.) to determine which file descriptors are available for I/O.
 
+Note that the file descriptor provided in
+.I si_fd
+is the one that that was specified during the
+.BR F_SETSIG
+operation.
+This can lead to an unusual corner case.
+If the file descriptor is duplicated
+.RB ( dup (2)
+or similar), and the original file descriptor is closed,
+then I/O events will continue to be generated, but the
+.I si_fd
+field will contain the number of the now closed file descriptor.
+
 By selecting a real time signal (value >=
 .BR SIGRTMIN ),
 multiple I/O events may be queued using the same signal numbers.