Most people get knocked out because they don't see the punch coming. The body blows would be the biggest problem, because even if you brace yourself there are no guarantees.
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On the afternoon of October 22, 1926, two McGill University students visited Houdini's dressing room. According to reports, Houdini was looking through his mail, when one of the students, J. Gordon Whitehead, asked Harry if he could indeed withstand any blow to the abdomen, as the magician had previously proclaimed. Harry responded that he could, if given time to brace himself, at which point Whitehead hit Houdini four times in the abdomen, under the impression that Houdini had indeed braced himself for the blows. Throughout the evening, Houdini performed in great pain. He was unable to sleep and remained in constant pain for the next two days, though he did not seek medical help. When he finally saw a doctor, Harry was found to have a fever of 102 degrees and acute appendicitis. He was advised to go to the hospital for immediate surgery. However, Harry decided to complete his show as planned that night.
By the time Harry arrived on stage, his fever had risen to 104 degrees. He was tired and in pain and his assistants often had to step in and offer help. Audience members reported that Harry missed his cues and seemed in a hurry. By the middle of the third act, Houdini asked his assistant to lower the curtain as he could not go on. When the curtain closed, Harry collapsed where he was standing and had to be carried back to his dressing room. He continued to refuse medical care until the next morning when Bess insisted he go to the hospital. Harry relented and had his appendix removed, however it had already ruptured and doctors did not have much hope for his survival. On October 31, 1926 surrounded by his wife and brother, Harry Houdini died.