1. Enhance (Far Too Many To Count)
http://www.youtube.com/watch?v=Vxq9yj2pVWk Of course this would be the number one pick. Countless movies, TV shows and other forms of procedural storytelling employ this device with reckless abandon. Any time somebody sees something recorded, be it on a photograph, security camera or anywhere else, there is an additional clue within the image. Fortunately for the detectives, it is easy to just repeatedly enhance something, until you can break it down microscopically get a look at somebody's fingerprint from the picture of their car taken from 50 feet away. Unfortunately, this is impossible. Have the writers of these TV shows never heard of the concept of a pixel? Apparently not, because it is a trope which has been used to death. And not only are the images themselves the focus of much attention, but any mirror or window which can cast a reflection, which also absurdly finds itself to be a key element of the detective's arsenal.
Honorable mention: Light speed jumps. Thoughts? Leave a comment below and check out the Science Episode of my podcast for more lists on bad science in film.