When people talk about slippery slopes, they're usually worried about one thing: if you start down the slope, you might slide to the bottom. But that's not the only risk, and it's sometimes not even the most serious. After all, if you can see the slope below you, there's a chance that you can inch downwards for a ways, being careful not to lose your footing, and stake out some ground in the middle of the slope. You might even get comfortable traversing a region of the slope, without even coming close to exploring the bottom.
To avoid slipping down, though, people often try to remain firmly planted on the apparently flat, and thus safe, ground at the top of the slope. Often far more unstable than it appears, this ground may erode and crumble beneath your feet, sending you tumbling towards the bottom with little chance to grab a handhold on the intervening slope. Or some external force, which is strongest up on the flat plain, may knock you over and make you skid down the incline.
For example, consider tax increases. Some people oppose any tax increase, even a slight increase on a small portion of the population, as a step in the wrong direction, as if it was likely to spread. But really, you're most likely to need widespread tax increases if you don't increase anyone's taxes now, and you just let the deficit grow until it gets too big to leave alone. Then the fiscal insecurity hits you and knocks you tumbling down the slope. If you had gone partway down the slope of tax increases, then the blow wouldn't have hit you with such force.
Can anyone think of better examples or other neglected problems with slippery slopes?