You often hear people claiming that the case is strong because there was an "eyewitness." It's becoming increasingly clear, however, that eyewitness testimony is often worse than useless. Modern DNA testing has exposed just how weak eyewitness testimony can be, as presented Radley Balko, in Reason:

Law and Human Behavior, false eyewitness testimony contributed to 77 percent of the 230 wrongful convictions exposed by DNA evidence over the last decade (the number of exonerations has grown since the study was conducted). These of course are only those cases for which DNA testing was available, which are usually murder and rape cases—crimes for which, generally speaking, there is also usually other evidence available. In crimes where investigators are more likely to rely only on eyewitnesses, robberies or muggings, for example, it’s likely that the problem is even more pronounced.