cau ‧ tion ‧ ar ‧ y /ˈkɔːʃən ə ri $ ˈkɒːʃəneri/ BrE AmE adjective [usually before noun]
giving a warning about what not to do
cautionary note/comment/words etc
Most observers were optimistic, yet some sounded a cautionary note.
cautionary tale (=the story of an event that is used to warn people)
a cautionary tale about how not to buy a computer