The bulk of g_assert_warning() is:

  g_log (log_domain,
         G_LOG_LEVEL_ERROR,
         expression 
         ? "file %s: line %d (%s): assertion failed: (%s)"
         : "file %s: line %d (%s): should not be reached",
         file, 
         line, 
         pretty_function,
         expression);

If expression == NULL, then the above reduces to:

  g_log (log_domain,
         G_LOG_LEVEL_ERROR,
         "filer %s: line %d (%s): should not be reached",
         file, 
         line, 
         pretty_function,
         expression);

The format string expects 3 arguments, but 4 are given.

According to  http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf
page 309 (327th of document):

    "If the format is exhausted while arguments remain, the excess
     arguments are evaluated (as always) but are otherwise ignored."

so the current code strictly speaking is legal. The downside is that you keep
seeing a false positive if you build with -Wformat-extra-args, which is a useful flag, as usually it just finds places where an argument was added, and the format string not updated.

Maybe we could change the code to the less cunning, but more compiler friendly:

  if (expression)
    g_log (log_domain,
           G_LOG_LEVEL_ERROR,
           "file %s: line %d (%s): assertion failed: (%s)"  
           file,
           line,
           pretty_function,
           expression);
  else
    g_log (log_domain,
           G_LOG_LEVEL_ERROR,
           "file %s: line %d (%s): should not be reached",  
           file,
           line,
           pretty_function);

?