Recently there was a case where the Finnish police arrested a person as a suspect for a robbery of a convenience store on basis of DNA evidence: the robber had dropped a plastic glove, presumably to hide fingerprints, on the floor of the store. Officers isolated DNA from the glove, searched archives, found a match with a person who had had some minor offense years prior, and brought him in for a questioning.
It turned out the DNA-matched first suspect was innocent when the police found a second suspect, who confessed to the crime and whose clothes fitted security camera footage of the robbery better. So how the first suspect's DNA arrived on the gloves at the crime scene? The investigators noticed he told he had visited the same mall where the second suspect (according to his testimony) had obtained the gloves prior to the robbery. Based on the security camera footage from the mall, the investigators concluded that the likeliest way the DNA of the first suspect ended up on the gloves the robber because they rode the same escalator.
Like this: https://images.cdn.yle.fi/image/upload//w_1200,h_1200,f_auto...
The lesson is, humans shed lot of skin cells around. The DNA from incriminating bodily fluids or similar is much better evidence than random DNA.
Source https://yle.fi/uutiset/3-11811155 and Google Translate: https://translate.google.com/translate?sl=auto&tl=en&u=https...