When commenting on the Gary Leiterman conviction, Dr. Theodore Kessis wrote, “It must be noted however that contamination errors have been documented where
no direct processing link between sample and contaminant have been established, raising the specter that a source of contamination can linger in a laboratory for some time.” I would bear this in mind when assessing the putative impossibility of contamination when there is a six-day gap between the time when some evidence was processed and when the knife DNA was
said to be processed.
Dr. Donald Riley wrote about DNA profiling based on polymerase chain reaction (PCR) technology, "When contamination occurs
there is rarely any way to confirm how it happened...In PCR's history, contamination has often led to false results, and erroneous actions have been taken based on those results."
(all highlighting mine)
http://www.scientific.org/tutorials/articles/riley/riley.html