First, the evidence such as blood drawn from O.J. was not handled the way it should have been, there was time when the blood was in polices possession that would have gave them plenty of time to go to O.J.'s home and spread it around his property and all over the driveway. Some of the blood found at O.J.s home did have a preservative in it that is added by police and is not in a person's blood. That shows that the blood had to have gone through the police before it got on O.J.s property, proving they planted it there.
Next, the main officer helping out the prosecution Mark Fuhrman was accused of being racist and of planting evidence to frame O.J.. He denied being racist under oath and he was later in the trial found to be lying, he was heard on a tape saying the word "nigger" and other racist things. He was also accused of planting a bloody glove at the Simpson home and planting bloody socks in O.J.s room. The blood on the socks was not consistent to what it should look like if they got blood on them when they were being worn, it was more like they were splattered with blood when they were off the defendant.
Most of the forensic evidence the prosecution had brought out was shown to be no good by Henry Lee, an expert in forensic science who was hired by the defense. Almost the whole case that the prosecutors argued was questionable and the jury's job is to