I've searched the forum without finding this trick, sorry if it's a repost.
With the servers lagging badly recently (EU servers anyways), we were a few guild mates cleaning up our banks since it was impossible to pvp / pve correctly due to the lag.
So after transferring some items we had in our personal banks to the guild bank (pots, food, enchanting mats, etc...), I asked them to put in some gold for the tanks' repair bills.
About 10s after two of my guildmates put some gold in the guild bank, the server apparently crashed.
When we relogged after they restarted it, my two guildmates still had they gold they put in the guild bank, and it was still in the guild bank too.
We tried this again the same day, and the day after, all running to the bank when the server began to lag badly and all putting some gold in it (in fact we put gold in it, then retake it, restash it, etc, untill the server finally restarts or crashes).
We didn't do it with large amounts to avoid getting banned, hell we just did it for fun, but we still managed to dupe around 800g in those 2 days.
So if you see a lot of people running to their gbank while your server is getting really unstable, then you'll know why they do so. ;p