ZOOKEEPER-1049 to 3.3.3 (I believe the same problem exists in 3.4/3.5 but haven't tested this) I'm seeing the following exception more frequently:
This is a long standing problem where we try to send a response after the socket has been closed. Prior to
ZOOKEEPER-1049 this issues happened much less frequently (2 sec linger), but I believe it was possible. The timing window is just wider now.