Must be a peculiarity of
$Version
(* Out[40]= "8.0 for Microsoft Windows (64-bit) (October 7, 2011)" *)
In 5.2 for Microsoft Windows x86 (64 bit) (June 20, 2005), which I still keep on my PC (you will remember; it is much faster doing Integrate than Version 8) I get the error message "Plot::plnr : Exp[-x] == Cos[x] is not a machine size real number at x = -1 ..." and an empty Plot which is correct.
