There was a warrior in our guild that had been with us for a year, had gone out of his way consistently to help the guild. We promoted him to Officer because he got along with people, seemed trustworthy, and had been with us through quite a lot, including a time when it seemed our guild was dying.
He was our main tank for our raids and let us know an hour before raid that he had been looking around for new guilds and was leaving. I immediately demoted him and his alts, to ensure he couldn't take anything from the guild bank, then I went to check the logs. 2 stacks of each kind of flask, half a stack of each kind of Greater shoulder inscription, Enchant Bracer - Greater Agility, a few of both types of Greater Spellthread, all taken around a week before telling us he was leaving, indicating it was very much planned out, and he apparently hoped that others would deposit/withdraw enough that it would be hidden. Priced on the AH the day he left, it was worth 15k without counting the flasks.
Admittedly, he did deposit a good amount of mats over the year, and I would have been fine with letting him have a stack of each flask and a few inscriptions had he mentioned he wanted to leave and asked about the items, but it was a massive betrayal of trust. His current guild leader has said that he doesn't care and that Dirk has a home there, so the best of luck to him, I hope he doesn't let Dirk build up false trust.
As I said to begin with, it is our fault for letting him have access to those tabs of the guild bank, I merely wish to warn others in case he decides to try and repeat the performance. He even changed his name afterwards. Before, he was Dbdirkish, now he is Ðirk. Alts that I'm aware of are Dirkswife, Dbcarl, Dbdirk, Dirkbag, Dirkish, and Drdirk.