Gossypiboma is the name given to a retained surgical sponge, pad, or towel (gossypium (Latin), cotton; + plboma (Swahili), place of concealment). Retained foreign bodies can cause either an acute inflammatory response or a chronic fibrinous response and so there is a very wide variation in the time course in which they are discovered, as evidence in a diverse array of case reports in the surgical and radiologic literature. The sponge in the above picture was found over a decade after an exploratory laparotomy for trauma as an incidental finding on a study performed for an unrelated reason.

Policies and procedures such as thorough surgical instrument counts and intraoperative x-rays are designed to prevent this complication, however retained surgical instruments continue to be occur at a rate estimated to be around one in 1500 laparotomies. It has been proposed that routine intraoperative x-rays are cost effective and should be performed for high-risk procedures despite correct instrument counts.

Surgical sponges are easily identified by the use of barium-impregnated threads woven into one side of the cloth.
Reference: N Engl J Med. 2003;348(3):228, 229-235
Credit: jhamilton