Second time today someone has reminded me of Hitchhiker's Guide to the Galaxy! In this case the Cathedral of Chalesm that illustrates the dangers of irresponsible time travel:

The cathedral was scheduled for demolition in order to build a new ion factory. However, due to delays in construction and a strict deadline for the start of ion production, the beginning of the project was extended so far back in time that the cathedral ended up never having been built in the first place. As a result, picture postcards of the cathedral suddenly became immensely valuable. And blank.