In physics, it is sometimes the case that the act of observation changes the thing being observed.  While it isn't that extreme in biology, a study in PLoS One may mean a genome biologists think is accurate may not be so.

Up to 20 percent of the nonhuman genomes they examined were contaminated with human DNA, they say, a discovery they made while looking for a human virus.   They found so much human-looking DNA in nonhuman genomes that they did a specific search for the common human sequence AluY, which occurs one million times in the human genome.   It could be skin.

Concern may also be overblown.   Dr. David Lipman, director of the National Center for Biotechnology Information, told the New York Times, “We don’t see the contamination she’s talking about.”  
Richard Gibbs, director of the Human Genome Sequencing Center at the Baylor College of Medicine, said that cross-species contamination “often occurs” and was easier for computers to filter out with the older sequencing methods. He said the PLoS One report did not add much new. “I don’t know of any good scientific work that has been compromised by cross-species contamination,” he said.
Citation: Longo MS, O'Neill MJ, O'Neill RJ (2011) Abundant Human DNA Contamination Identified in Non-Primate Genome Databases. PLoS ONE 6(2): e16410. doi:10.1371/journal.pone.0016410