After running xscreensaver on my Debian Jessie system with XFCE, the
number of "unknown" X clients listed in xrestop grows until I get
"Maximum number of clients reached" when trying to start a new
application.  I usually takes a few days from when I start X to when I
get the "Maximum number..." error.  When I don't start xscreensaver, the
problem doesn't occur.

I'm running xscreensaver with the following screensavers enabled:
   * Carousel
   * GLslideshow
   * Photopile

Trying different combinations of those screensavers doesn't fix the
problem.