The slime pit was similar to when Rambo had feces dumped on him when he was captive by the Vietnamese during the war, and is yet another traumatic experience that sticks with John Rambo still.
John Rambo, along with Banks, is captured by the North Vietnamese Army and is promptly taken to a POW camp located in Northern Vietnam. He is stripped of all of his clothing except underwear and lowered into a drainage pit filled with raw sewage, garbage, urine, dishwater and mucus. He is held by having his arms strapped to a bamboo stick, giving him much excruciating pain. Rambo is spit at by Lieutenant Tay while he is in the pit and suffers multiple leeches clinging to his skin. Rambo is lifted out of the wastewater by the orders of Russian Colonel Podovsky, and Rambo is left almost completely coated in human and animal waste. Podovsky later uses Rambo's own knife to slice a leech from his body in a painful manner.