A couple of quotes from Wikipedia on the OJ Trial:

"What should have been the prosecution's strong point became their weak link amid accusations that bungling police technicians handled the blood samples with such a degree of incompetence as to render the delivery of accurate and reliable DNA results almost impossible."

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"In March,[LAPD detective] Fuhrman testified to finding blood marks on the driveway of Simpson's home, as well as a black leather glove on the premises, which had blood of both murder victims on it as well as Simpson's. Despite an aggressive cross-examination by F. Lee Bailey, Fuhrman denied on the stand that he was racist or had used the word "nigger" to describe black people in the 10 years prior to his testimony. But a few months later, the defense played audio tapes of Fuhrman repeatedly using the word – 41 times, in total. The tape had been made in 1986 by a young North Carolina screenwriter named Laura McKinny. She had interviewed Fuhrman for a screenplay she was developing on police officers. The Fuhrman tapes became one of the cornerstones of the defense's case that Fuhrman's testimony lacked credibility.

In September, Fuhrman was called back to the witness stand by the defense to answer more questions about the discovery of the blood marks and leather glove that he supposedly found on Simpson's property hours after the murders took place. When questioned by Simpson attorney Gerald Uelmen, Fuhrman, with his lawyer standing by his side, pleaded the Fifth Amendment against self-incrimination to avoid further questioning after his integrity was challenged at this point."