Don't drink the Kool-Aid!

From Wikipedia:

It is also now closely associated with the 1978 cult mass-suicide/murder in Jonestown, Guyana. Jim Jones, the leader of the Peoples Temple, convinced his followers to move to Jonestown. Late in the year, he then ordered his flock to commit suicide by drinking grape-flavored Flavor Aid laced with potassium cyanide. In what is now commonly called the "Jonestown Massacre," a large majority of the 913 people later found dead drank the brew. (The discrepancy between the idiom and the actual occurrence is likely due to Flavor Aid's relative obscurity versus the easily recognizable Kool-Aid.) The precise expression can be attested in usage at least as early as 1987. One lasting legacy of the Jonestown tragedy is the saying, "Don't drink the Kool-Aid." This has come to mean, "Don’t trust any group you find to be a little on the kooky side," or "Whatever they tell you, don't believe it too strongly." The phrase can also be used in the opposite sense to indicate that one has blindly embraced a particular philosophy or perspective (a "Kool-Aid drinker", or, as a cynical response to a fanatical claim, "sounds like someone's been drinking the Kool-Aid!").