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Cheryl's decision not to vote off one of the acts from her category is not unprecedented

Wikipedia:

Osbourne's walkout
During the results of the first live show on 20 October, judge Osbourne walked away from the panel after it was revealed that both the bottom two acts came from her category. This left the three remaining judges (Cowell, Minogue and Walsh) to decide which of the bottom two would leave the contest. When presenter Dermot O'Leary asked Osbourne for her vote she claimed to have left the show, saying, "I'm out—gone". It later emerged that Osbourne was dissatisfied with the last-minute rescheduling of the programme from 17:45 to 17:30, claiming that fewer people saw the performance of her act, Kimberley Southwick, as she was first on stage. After a considerable amount of media coverage, including Paul O'Grady apparently convincing Osbourne to return live on his chat show, it was confirmed on 23 October by Osbourne's spokesman that the judge would return to the show. A newspaper source explained that leaving prematurely would have been breach of contract and the legal repercussions would have made it in Osbourne's interests to return to the panel.