Apparently it is possible to create fake blood and saliva samples containing DNA from a person other than the donor, using a standard technique called whole genome amplification.
And, to take it a step further, it is also possible to construct a sample of DNA to match someone’s profile without obtaining any physical tissue from that person.
The latter relies on having access to the DNA profiles stored in law enforcement databases. Our DNA is stored as “a series of numbers and letters corresponding to variations at 13 spots in a person’s genome. Scientists can clone tiny DNA snippets representing the common variants at each spot, creating a library of such snippets.” One can then “prepare a phony DNA sample matching any profile by mixing the proper snippets together.” [science.slashdot.org]