What is good for the Magnificent Seven names is not necessarily good for the rest of the equity market,” one strategist said, as stocks suffered their worst week since May.

But there is a non-statistical basis for concern: an ominous parallel with the financial environment that prevailed in the weeks leading up to the October 1987 stock-market crash. On that day — Black Monday — the Dow Jones Industrial Average dropped 22.6% in a single session.