Steps to reproduce:
1. Compile this program with debugging info

#include <gdkmm/pixbuf.h>
#include <gtkmm/main.h>
#include <glib/gmessages.h>

int
main (int argc, char* argv[])
{
    Gtk::Main loop (argc, argv);

    GError* gerror = 0;
    Glib::RefPtr<Gdk::Pixbuf> pb = Glib::wrap (gdk_pixbuf_new_from_file ("/tmp/nosuch", &gerror));

    if (gerror)
    {
      printf ("%s:%d: Error: %s\n", __FILE__, __LINE__, gerror->message);
      std::auto_ptr<Glib::Error> e (::Glib::Error::throw_exception (gerror));

      // Never reached
      g_assert_not_reached ();
    }
    else
    {
        printf ("%s:%d: No error\n", __FILE__, __LINE__);
    }

    return 1;
}


2. Break on main.cc:19
3. Observe nonsensical output:

main.cc:15: Error: Failed to open file '/tmp/nosuch': No such file or directory
main.cc:23: No error

Stack trace:
There isn't a stack trace at the point of failure; the program just jumps into the wrong logic paths and crashes later because error-throwing operations appear to have succeeded.

Other information:
This code is basically lifted from the sources to Gdk::Pixbuf::create_from_file(), for the purposes of instrumenting it with debugging output.

Apologies for using the Pixbuf C++ binding type from gtkmm as a demonstration of the bug. I didn't find any operations which ought to provoke the same Glib::FileError instance in the core glibmm API itself.

At any rate, I don't think that the Gdk::Pixbuf code is at issue here; I can witness the Glib::FileError::throw_func() call return and produce the expected instance of Glib::FileError. The strangeness begins when Glib::Error::throw_exception() attempts to return that auto_ptr to the calling code (here, main.cc:16.