For many years I have used "ggg" in notes, papers, and code of all kinds in order to mark places that need future attention. The letters don't appear in ordinary English or phonetic transcription of Chinese, so they're easy to find without error.
Alas, these three letters have become popularized by the sex columnist Dan Savage. (See http://www.thestranger.com/seattle/SavageLove?oid=167448, accessed 20130929.) And so my innocent and neutral marker has taken on a meaning heavy with implications I never anticipated.