Alright i did search for any threads related to this and came up with nothing, so here is my first attempt to contribute to this site.
While going through my guild bank today i saw a few LW patterns i could use on my alt so i tried to take them out not realizing that i didn't have permission.
After i figured out i couldn't withdraw anything i was going to put some stuff in the bank.
i dragged a stack of potion from my bags into the GBank over the pattern just by chance and it exchanged the items.
my potions into the bank and the pattern into my bags.
as many of you may be thinking, this is a very good way to steal items from a guild.
Hope this isn't a repost and have fun with it =]