No, after sorting the individual materials will not be mixed together in any case. Today, as each ton of waste disposed of in a landfill is charged, no one is interested to mix sorted wastes again. In the Czech Republic there are enough processors, who can convert sorted waste either into secondary raw materials and place it on the market, or into certain final products that have become already common in daily use.
Unfortunately, here and there are people, who are able to contaminate the whole charge of sorted waste with undesirable admixtures, such as, most commonly, mixed municipal waste. If it happens and the whole content of the container of sorted waste has been contaminated so much that it cannot be handled as sorted waste, it must not be loaded on the collection vehicle in order the whole load to be prevented from spoiling. Such contaminated waste should be handled as mixed waste and is to be transported to either an incineration plant or a landfill for disposal.
However, one should note that such incidents are relatively rare and people try to sort their waste out with care.