Hundred Flowers, then the Anti-Rightist campaign
An invitation to criticise, then 550,000 labelled — and twenty years to rehabilitation.
What happened
In 1956 the Party invited criticism — "let a hundred flowers bloom, let a hundred schools of thought contend." When the criticism came, and cut deeper than expected, the invitation was reversed. The 1957 Anti-Rightist Campaign, directed by Deng Xiaoping, labelled roughly 550,000 people as "rightists."
The results
Many were sent to labour camps or internal exile; some did not return for twenty years, and most were not formally rehabilitated until after 1978. An entire layer of intellectuals learned that candour — even when the state itself had asked for it — was a trap. The country lost the critical voices it would badly need two years later, as the Great Leap famine built with almost no one left willing to report it.
Why it happened
Whether the invitation was a deliberate flushing-out — Mao later said he had "lured the snakes out of their holes" — or a panic once the criticism bit, the lesson is the same: in a one-party state the machinery for ending a political purge sits entirely in the hands of the people running it. There was no court to appeal to, no opposition that gained from the scandal, and no press outside the Party. So the correction waited for Mao to die.