Degradation is usually caused when bacteria, mold and mildew grow on a sample. This can lead to inconclusive results. Contamination can lead to false or un-interpretable results. This happens because the PCR reaction will preferentially work on fresh DNA over older samples. So, even if a small amount of fresh DNA comes into contact with a crime scene sample that has been exposed to the environment , it is possible for that fresh DNA to give results along with or instead of the crime scene sample.
Contamination can happen both in the field or in the laboratory.
Degradation that occurs at the crime scene , before collection , cannot be helped. The DNA testing methods utilized now can more often then not obtain reliable results from those. However, degradation that happens to due improper collection and storage is inexcusable. By just adhering to a few simple practices this type of degradation can be avoided. Use paper bags, envelopes, etc. to package items.