Depending on the jurisdiction, all analysts will work in the lab. Well, like everything else in forensic science - it depends. You have to get a medical degree and then some more specialized training to do this. There is an ever-increasing pressure to have an advanced degree as well. And also a way to get even some more detailed information from there.
Contamination is defined as any substance placed into or onto evidence by a responsible party.
So one answer to your question might be, if a substance contains human DNA and more specifically DNA from a reference sample that might give a false positive result. That is the worst case scenario in DNA anyway. Both at the crime scene and in the laboratory, we take extreme care to minimize all kinds of contamination, but especially this type. In that case, it may be a mixed sample, and the interpretation would be difficult, but it would not be considered contaminated. One example of contamination would be improper storage of biological samples where the evidence is packed wet and airtight, allowing microbes to grow.