This 

<quote>

   Second, consider the comment line.  Did you notice that it ends oddly,
   with a "/"?

       // What will the next line do? Increment???????????/
                                                          ^

   Nikolai Smirnov writes:

      "Probably, what's happened in the program is obvious for you but I
      lost a couple of days debugging a big program where I made a
      similar error.  I put a comment line ending with a lot of question
      marks accidentally releasing the 'Shift' key at the end.  The
      result is unexpected trigraph sequence '??/' which was converted to
      '\' (phase 1) which was annihilated with the following '\n' (phase
      2)."  [1]

   The "??/" sequence is converted to '\' which, at the end of a line, is
   a line-splicing directive (surprise!).  In this case, it splices the
   following line "++x;" to the end of the comment line and thus makes
   the increment part of the comment.  The increment is never executed.

   Interestingly, if you look at the Gnu g++ documentation for the
   -Wtrigraphs command-line switch, you will encounter the following
   statement:

      "Warnings are not given for trigraphs within comments, as they do
      not affect the meaning of the program."  [2]

   That may be true most of the time, but here we have a case in point --
   from real-world code, no less -- where this expectation does not hold.

</quote>

is extracted from Herb's GTW #86 -- full message appended below.
I would suggest we warn for trigraphs in comments.

-- Gaby