"An entomologist at New College, Oxford ("New" because its only a few centuries old), discovered beetles infesting the oak beams supporting the roof of the Great Hall. It was fairly urgent that these be replaced before the roof collapsed--but anyone who has looked at the price of oak lately can tell you that this was not something the college budget was prepared for.
Since oak from a commercial supplier was out of the question, someone suggested that the college Forester be sent for. His job was to administer the various scattered tracts of land that had been deeded to the college when it was founded. The trustees hoped he might know of suitable trees on college land.
It turned out that there was indeed a suitable stand of mighty oaks. They had been planted when the college was founded, and down the centuries each Forester had told his successor: "You don't cut those oaks; those are for when the beetles get into the beams in the Main Hall."