Comment by devilbunny

Metal has the marvelous quality of showing up well on X-ray, and clothing has the marvelous quality of never getting cancer from X-rays.

Pieces of cotton do not show up on X-ray, and humans do get cancer from too much ionizing radiation.

Of course miscounts happen sometimes, and sometimes both the initial and final counts are one short of the true number, but the vast majority of undetected retained items are sponges made of cotton. Not tools, not even tiny needles. That’s why there is a radiopaque strip embedded in the sponges intended for use during surgery.

It is not perfect, and cannot be.