The latter. Partial matches are one concern (where someone comes and asks you questions because they think one of your relatives that you've never met may have

The latter. Partial matches are one concern (where someone comes and asks you questions because they think one of your relatives that you've never met may have committed a crime), but laboratory errors and general investigational misconduct are the big one (e.g. http://www.cbsnews.com/news/dna-testing-foolproof/ http://www.cbsnews.com/news/dna-testing-foolproof/ from 2003, or a Brandon Mayfield like situation with DNA instead of fingerprints).