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.
Yes you are right, using the command-s function causes Autocad to crash, when a command reactor is loaded. I eventually found a description of this function in 2013 help. It says that the function runs in a temporary command processor separate from the main Autocad command processor, and gives an example of how improper use might crash Autocad. - using (command-s "U") while an ordinary command is active. It would appear that they forgot to consider how the function would handle command reactors. Ian