As I Heard Them, Vol. 1, p. 225, no. 41
In the course of a farbrengen at his mother's house, the Rebbe related that a chossid dreamt that his previous Rebbe had told him, "In the coming year one of your children will pass away." When the chossid went to his Rebbe for Rosh Hashanah, the Rebbe told him, "Tracht gut vet zein gut" (Think positively, and it will turn out well). But, unable to shake off what the previous Rebbe had told him, the chossid was deeply apprehensive and returned home.
On Simchas Torah, when everyone was dancing, he stood to the side and wept. But then came the thought: "Nonetheless, it's Simchas Torah!" And he jumped into the dancing.
When he went back for yechidus after Sukkos, the Rebbe asked him, "What did you do on Simchas Torah?" He described what had happened, and the Rebbe told him that his exuberant "Tracht gut vet zein gut" nulli-fied the decree.