The "slowly boiling frog" is a good metaphor for failing to perceive a threat when change is gradual enough, but according to Wikipedia it is not real:
> Modern scientific sources report that the alleged phenomenon is not real. In 1995, Professor Douglas Melton, of the Harvard University Biology department, said, "If you put a frog in boiling water, it won't jump out. It will die. If you put it in cold water, it will jump before it gets hot—they don't sit still for you." Dr. George R. Zug, curator of reptiles and amphibians at the National Museum of Natural History, also rejected the suggestion, saying that "If a frog had a means of getting out, it certainly would get out. [1]