The entire site became inaccessible after enabling XHProf because of an incorrect entry in the "xhprof directory:" field. What appeared instead was an error display indicating the file was not found on line 487 in devel.module. To recover I had to disable XHProf from phpMyAdmin (devel_xhprof_enabled in the variable table). Upon disabling I found the "White Screen of Death" on every page of my site. This is because most of the entries in the theme registry had been wiped out. To get the site back up I had to get the theme registry rebuilt by executing "drush cache-clear" from site root and select every cache to be cleared.
The devel module needs to check for the presence of the file when the path is entered in the "xhprof directory:" field. If not present it should warn the user and disable XHProf. If the program does branch to the vicinity of line 487 in devel.module while the entry at "xhprof directory:" is incorrect devel needs to recover gracefully from this error so the site will remain accessible.
Also, the example paths below the "xhprof directory:" field which are now:
Location of the xhprof source code on your system, usually somewhere in /usr/local/share or /usr/share, include the leading forward slash.
need to be changed to:
Location of the xhprof source code on your system, usually somewhere in /usr/local/share/xhprof or /usr/share/xhprof, include the leading forward slash.
to alert the user that the "xhprof" directory needs to be included at the end of the path.