Using the official Ubuntu PPA release: 92792-c3ff671-emacs-25.2~ubuntu16.04.1
I want to rectify the bug report because it does
not happen with emacs -nw but with emacsclient -nw.
Only when using the middle click button, not with urxvt Insert-shift facilities. Each and every mouse motion (scroll wheel, right, left click...) shows garbage in the terminal.
I have also tried on version 25.1.1, getting the
same result.