I would say the media, through drama and dramatizations, have oversimplified the criminal investigation process.  The structure of most crime shows is that within thirty minutes, all crime scenes can be investigated and thoroughly understood, pointing to the appropriate culprit.  I think this oversimplification creates the understanding that nearly anyone, even those who lack a law enforcement background, can play junior Gumshoe detective and figure out the elements of a crime.  People who have forgotten loved one's birthdays apparently know everything there is to know about DNA, its reliability, and its effectiveness in identifying suspects.  I think this is probably where the media has played a significant role in helping to form our perception of criminal investigations.

You'll notice that in most television crime dramas, the criminals always seem to confess under interrogation.  This happens much more rarely than we would think.

Watch any one of the 28 different CSI programs and you would think that we are able to forensically prove and decipher every single criminal case, while everyday law enforcement does not have the resources, time or experience to conduct those types of investigations, not to mention that the physical evidence left behind is often compromised, incomplete or useless.

Law enforcement is a very inexact science.  Media leads us to believe otherwise.

Many times the media only reports one side of the story. There is numerous evidence involved in a criminal investigation that the public does not even know about. On the news we only hear the worst details possible. The purpose of the news is of course to keep us informed but in doing so they must also keep our attention or we will turn to a different news station. One way that the media does this is by reporting the shock factor of the investigation. Unfortunately, this leaves out details that may otherwise tell us a different story to the crime.

I think there are a couple of ways in which media depictions have affected our perceptions of criminal investigations.

First, I think we tend to believe that these investigations are in some way completely foolproof.  We have come to believe that the police always "get their man" because of how well they investigate.  This seems to be different from reality since in reality lots of people get away with crimes.

Second, I think we have come to believe that scientific evidence and high tech tools tend to be the things that solve cases.  This tends to make us think that everything can be solved when that's not really true.