Via Wikipedia’s article on embalming:
Pius XII’s (pope 1939–1958) botched embalming by a charlatan doctor—which only sped up the rate of decomposition—led to his body turning black and his nose falling off while lying in state, and the body disintegrated in the coffin. The Swiss Guards stationed around Pius XII’s body were forced to change shifts every ten to fifteen minutes since the body’s odor caused some guards to pass out. The doctor who performed the embalming had also taken photos of the Pontiff in his death throes and intended to sell them to tabloids. The Italian tabloids refused to buy the photos, and the doctor was banned from entering the Vatican City-State by John XXIII, who furthermore prohibited any photography of a deceased Pope until the body is properly vested and laid out.
And from Slate’s article ‘Why Didn’t They Embalm the Pope?’ (written after the death of John Paul II):
John Paul II will not be the first pope to decompose in public. In August of 1978, the body of Paul VI “took on a greenish tinge,” and fans were installed in the Basilica to disperse the smell. Twenty years earlier, a maverick doctor persuaded the Vatican to let him try an experimental embalming technique on the body of Pope Pius XII, with disastrous consequences—the body turned black and disintegrated in the casket.