It seems that the error is triggered by the use of command-s within the routines error handlers.
A search tells me that command-s appears in these files so presumably they will all be affected.
It seems that command-s was introduced in AutoCAD 2012. The MVSETUP and EDGE lisp routines which shipped with 2011 and earlier use 'command' and these versions do not trigger the fatal error. However, the use of command-s on 2012 and 2103 when a CommandWillStart reactor is loaded will trigger a fatal error.
Partial error routine from Edge.lsp (2011)
(defun edge-er (n) (if (/= s "Function cancelled") (princ (strcat "\nError: " n)) ) (command) (command "_.UCS" "_P")
Same section from Edge.lsp (2013)
(defun edge-er (n) (setq *error* lisp-er) (if (/= s "Function cancelled") (princ (strcat "\nError: " n)) ) (edge_setCmdEcho 0) (command-s "_.UCS" "_P")
If you are running 2012/2013 then load the reactor from my original post and then use this command and you will get a fatal error:
(command-s "_.line" "0,0,0" "10,10,0" "")
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