I experienced repeatable crashes of x11/rxvt-devel with sysutils/synergy
on amd64, saying "rxvt: XError: Request 18 . 0, Error: 2".  It always
happened when a selection was active on the rxvt and the mouse pointer
moved off from the X display to the synergy client display.

Here is the cause:  (screen.c line 3549~ inside rxvt_selection_send())

    } else if (rq->target == r->h->xa[XA_TIMESTAMP] && r->selection.text) {
        XChangeProperty(r->Xdisplay, rq->requestor, rq->property, XA_INTEGER,
                        (8 * sizeof(Time)), PropModeReplace,
                        (unsigned char *)&r->h->selection_time, 1);

On amd64, sizeof(Time) is 8, but 64 is invalid for XChangeProperty(),
and it generates BadValue error.  It should be 32 instead, which means "long".

Fix: 

Change (8 * sizeof(Time)) to 32 (more appropriate macro available?).
How-To-Repeat: Configure a synergy client and a synergy server.  Run rxvt-devel on the
server side.  Select some text on the rxvt by a mouse.  Move the pointer
over to the client side.

(I couldn't find more convenient way to invoke rxvt_selection_send)