The flip answer to that is: so fire them! Vote 'em out and replace them with people who will follow the rules. Not so easy, I know. I've been involved in a couple of organizations that had that problem, and it's easier for most members, when discovering the wool over their eyes and misuse of their funds/efforts, to just leave. So if a museum has a problem keeping members, or if high-profile members or groups of them leave without a good reason, it's a danger sign.

In some cases, more serious efforts may be needed. If bylaws are ignored, or changed without input/vote of the members, there may be legal concerns. In the worst case, a lawsuit might be needed to straighten things out (or to terminate the organization if it's bad enough).

Mission statements, vision statements, etc. are good, and are needed for any organization. But be careful - as useful as they are, they have no force unless they are reflected in bylaws or other binding documents. And of course "mission creep" can happen which ends up with the focus being on something different from what the original purpose was. If that happens openly, is fully discussed, and is supported by amended bylaws, fine; if not, the problems we've seen discussed here and elsewhere can happen.