What is The slippery slope fallacy?

Slippery slopes are so tricky to negotiate that even the first timid step upon them sets you sliding all the way to the bottom. No one ever goes up a slippery slope; they are strictly for the descent to disaster. The fallacy is that of supposing that a single step in a particular direction must inevitably and irresistibly lead to the whole distance being covered. There are cases in which one step leads to another, and cases where it does not. It is not a fallacy to suppose that after the first stride, further steps might be taken towards unpleasant consequences, but it is usually an error to suppose that they must.

There is a limited class of cases in which someone is doomed after the first step; stepping off a skyscraper is one of them. But in most life situations there is a choice about whether or not to go further. Those who oppose progress, however, often use the slippery-slope argument to suggest that any reform will lead inexorably to unacceptable results.

I oppose lowering the drinking age from 21 to 18. This will only lead to further demands to lower it to 16. Then it will be 14, and before we know it our new-boms will be suckled on wine rather than mother's milk.

The point is that the factors which lead to the arbitrary drinking age of 21 might change. There is nothing which suggests that they must keep on changing, or that society must keep on responding.
The slippery slope basically argues that you cannot do any-thing without going too far. This belies human progress, which has often been made by taking short steps successfully where longer ones might have been ruinous.

If we allow French ideas on food to influence us, we'll soon be eating nothing but snails and garlic and teaching our children to sing the Marseillaise.
(It might beat pizza and chips, though.)

In some cases there is a point of principle at stake which, once yielded, allows anything. This is not so much a slippery slope, however, as a vertical drop. The story is told of a dinner-table conversation between the dramatist George Bernard Shaw and a pretty lady:

'Would you sleep with me for a million pounds?'
'Why yes, I would. '
'Here's five pounds, then. '
'Five pounds! What do you think I am?'
'We've established that. Now we're talking about price.'
(Shaw was correct, but this is not a slippery-slope argument which would have led the lady to immorality in stages. Once the principle was conceded, the rest was bargaining.)

On a slippery slope ruin is reached in stages. The fallacy introduces the irrelevant material of the consequences of more far-reaching action in order to oppose the more limited proposal actually made.

Use the fallacy yourself to oppose change. There is scarcely any proposal which would not lead to disaster if taken too far. They want to charge people for admission to the church bazaar, but you point out that if this is conceded they will charge more next year, and more after that, until poorer people will be unable to afford to get in. The fallacy works best on pessimists, who are always ready to believe that things will turn out for the worse. Just assure them that if they do anything at all, this is almost certain to happen.