How Did That Make It Through Peer Review?

Yet, I suspect these kinds of situations are relatively rare. Having been involved in enough papers, and, yes, being party to papers where I didn’t catch something in the review or editorial process, I have the ultimate answer:

Reviewers, editors, and authors are human.

What I mean by this is that scientific papers are complex beasts. A single manuscript may weave together disparate groups of organisms, unfamiliar pieces of anatomy, far-flung reaches of the globe, and multiple statistical techniques. A typical paper is usually seen by a single editor and two to four reviewers. It is extremely unlikely that every facet of the paper will be seen by an appropriate expert on that given facet. How likely is it that every error will be caught and addressed?

From How Did That Make It Through Peer Review? | PLOS Paleo Community