Boil the frog slowly
The boiling frog story is a widespread anecdote describing a frog slowly being boiled alive. The premise is that if a frog is placed in boiling water, it will jump out, but if it is placed in cold water that is slowly heated, it will not perceive the danger and will be cooked to death. The story is often used as a metaphor for the inability of people to react to significant changes that occur gradually.
The boiling frog story is generally told in a metaphorical context, with the upshot being that people should make themselves aware of gradual change lest they suffer eventual undesirable consequences. This may be in support of a slippery slope argument.
Hear a short 4 minute audio about "boil the frog slowly".