Everyone should be aware of a guild bank exploit that Blizzard is aware of.
Known issue with Guild Bank withdrawal limits
At this time, Guild Bank withdrawal limits for a given character are not reset when they join a new guild. Thus, whatever limits were applied to that character in their former guild will be carried over, and must be manually readjusted by the Guild Master in order to ensure the desired settings.—Solution ID: 21684.
Colsprings @ Anvilmar explains how the exploit works at the following thread. Tell your guild master and leaders of this exploit to prevent further problems with guild applicants who may be potential exploiters. An upcoming patch will resolve this issue.