DNA evidence is far from being the infallible crime fighting tool that we all think it is.
I have heard of instances where criminals have taken DNA material from others and left it at crime scenes.
Drug addicts have been known to take another addicts used syringe (which contains blood - DNA material) and deposit this DNA material at the scene of a burglary.
Now I'm sure most of you wouldn't give a stuff about drug addicts leaving each others DNA at crime scenes, but what if criminals collect DNA material from other sources?
Do you have any idea how much of your own DNA you leave lying around during an average day?
On the train. On the bus. In the pub. At work.
It just needs a criminal to pick up a few of your hairs after you've left the pub and deposit them at a crime scene and you're nicked! And if you don't have an alibi, what are a potential jury going to think? Your DNA is at the scene of the crime, you have no alibi = GUILTY?!?!
All of the above is happening right now - FACT.