The 1978 World Chess Championship was between Anatoly Karpov, the champion, and Viktor Korchnoi, the challenger.
According to Wikipedia: "When Karpov's team sent him a blueberry yogurt during a game without any request for one by Karpov, the Korchnoi team protested, claiming it could be some kind of code. They later said this was intended as a parody of earlier protests, but it was taken seriously at the time."
Karpov eventually retained the title, winning the match by 16½ points to 15½.
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