Does anyone know how MessageBox(...) could fail silently?

MessageBox(g_hMainhWnd, buffer, "Oops!", MB_OK | MB_ICONERROR);

ShellExecute(0, "open", "http://intranet/crash_handler.php", NULL, "", SW_SHOWNORMAL);

For a little context, this code is called inside our own exception handler, which was registered with SetUnhandledExceptionFilter()

Most of the time, I see the message box, and then it launches a web browser.

However, I have an exe, which as far as I'm aware uses this exact code, and it successfully launches the web browser, but I do not see the message box first.

Thanks

Tim