This is reproducible both with Xorg-7.2 and with the latest released X components. Xnest -noreset -ac -geometry 640x480 :1 &>/dev/null & Xnest -noreset -ac -geometry 512x384 :2 &>/dev/null & Xdmx -display :1 -display :2 :3 <snip output> Xdmx: xcb_xlib.c:41: xcb_xlib_lock: Assertion `!c->xlib.lock' failed. -- Alexander E. Patrakov