If Evolution doesn't make mistakes, then why did Human loose their tail?

Evolution depends on mistakes made in nucleobase transcription. Guided only by selective pressures, it builds upon previous morphology in a way that further ensures many mistakes, from near sighted vision to bad backs in humans. And the loss of the tail did not occur in early humans. It occurred in our great ape ancestor, Ramapithecus, 18 million years ago (all great apes are tail-less).