A routine slackpkg upgrade-all
on Slackware64 13.1 upgraded xrdb to xrdb-1.0.9-x86_64-1_slack13.1 and resulted in breaking urxvt. The cause was hard to find; it turned out to be a trailing space in ~/.Xresources on custom line
which sets up Super+E to call the sub paste in the perl file "clipboard".
IDK whether other X resources or programs would be more tolerant of trailing space(s) and whether this is Slackware-specific (unlikely, given Slackware's philosophy) but it is such a difficult problem to identify I thought it worth this "heads up".