no such thing as a perfect witness. in fact, eye witness testimony tends to be grossly inaccurate. it has been proved time and time again in psychology studies that people A) perceive VERY little of anything that happens around them B) can be tricked into thinking they saw something simply by implied suggestion (i.e. "Did you see the woman in the red jacket?" when there was no woman in a red jacket in the video they just watched, but they will say they saw her.)
Unfortunately, most people still trust eye witness testimony, and they'll trust cops even more.