Slippery Slope occurs when one is proposed to start a chain reaction to something undesireable. The weakness in this argument strategy is based in the credibility of the chain of events.
"If we permit homosexuals to marry, then the next thing you know, we'll be legalizing pedophilia and rape."
"If we permit euthanasia of people with terminal cancer, pretty soon we will be permitting the killing of seniors, and the sick and the handicapped, and eventually only those deemed 'perfect' will be allowed to born."