I'm going to give you the benefit of the doubt and assume you're not a troll.
Times where that happens are exclusively when the previous systems have been overthrown by revolution leaving a power vacuum that bad faith actors have taken advantage of to assume dictatorial power for themselves and claim they are communist, make awful decisions not backed by experts or agreed upon by the masses, that leads to mass death, usually by famine.
We also have examples of countries democratically going socialist, but can't comment on their long term success as they usually get disposed by American backed death gangs or CIA organised Coups.