A slight malfunction, miscalculation, or a random collision with any of billions of pieces of space debris could have resulted in the probe, and its deadly plutonium, reentering earth's atmosphere and thereby incinerating its deadly cargo. NASA claimed the chance of such an accident resulting in a plutonium release was one in one million. Many have never agreed with that assessment. Cassini contains over 72 pounds of a radioactive substance called Plutonium Dioxide. The isotope of plutonium used in Cassini, Pu-238, is especially dangerous because of it's rapid rate of radioactive decay.
Ingestion is much less hazardous but is still far from safe.
In the June 1997 EIS they reduced these values by about an order of magnitude. The science about Saturn which Cassini seeks to solve is not worth the cancers Cassini can cause, especially when the same science could be achieved safely, if not this year, certainly soon. Saturn isn't going anywhere and if we take care of our environment, neither are we. Cassini is part of the problem, it is not part of the solution. Cassini contains more plutonium than NASA had ever launched into space before.