I have been reading about this for a while, and it is actually not as rare as you may think. There is something in the law called the "cy pres" doctrine, from Wikipedia, generally meaning: "When the original objective of the settlor or the testator became impossible, impracticable, or illegal to perform, the cy-près doctrine allows the court to amend the terms of the charitable trust as closely as possible to the original intention of the testator or settlor to prevent the trust from failing." Cy pres covers more ground than just trusts or wills, but you get the idea.
For people who do want to make a donation to charity, they have to be dam'n clear that cy pres will not be utilized (get a lawyer on this), and even then, a court will sometimes bend what the donor wants in order to preserve the charity. Certainly, do not sign something the charity/museum gives you because the odds are pretty good the paperwork will allow future "de-acquisition." That's because lawyers write things to help their clients, like, the charity/museum.