The last dehydrating alcohol must be 100% to ensure that absolutely no water will be carried into the following clearing agent. If the clearing xylene is contaminated with water it will appear milky-white and the slides will not clear.
Once the slides have gone into a contaminated clearing agent the only remedy is to bring them backwards to water and then dehydrate and clear them again in fresh solutions.
The xylene in the coplin jar on the left side of the image is contaminated with water. Note how cloudy it is compared to the one on the right which is not contaminated.