Hi, let's say that 1 million people all have the same genetic marker. If the fingerprint looks for this marker, the results are no longer unique, like a fingerprint. Rather, they are looking at only one part of the fingerprint that may be shared by millions of people. This would not be a random error, which the passage tells us would not be an issue, but rather one caused by the fact that the thing we are treating as unique is not actually unique.