Fallacies come in pairs sometimes.
Let’s take two twains of examples:
1. Slippery slope and runaway train. The slippery slope fallacy is committed when your argument is solely based on accusing the other side for going too far: e.g. it is a fallacy to claim that higher taxes will simply make everyone as poor as everyone else. The runaway train is committed when you actually go further than you argued for: e.g. it is a fallacy to base your argument on higher taxation solely on that no-one should have more than his neighbours.
Thus, if you are keen to mix metaphors, you may accuse someone of standing on a slippery slope only if they are already aboard a runaway train.
2. Bogus dilemma and Thatcher’s blame (latter was dubbed, as far as I know, by Madsen Pirie). The fallacy of bogus dilemma is committed when you present a dilemma when there is none: we will either have to lower our taxes, which will make the poor suffer, or we will have to raise our taxes, which will halt the economy, or, finally, we may keep the taxation as it is, and stay in the mess we are in. (Fallacy is in the over simplification of the issue of taxation.) Thatcher’s blame, on the other hand, is a failure to recognise a genuine dilemma. If there are no other options (lowering, raising, or keeping it as it is), no one can be blamed for the making of choice between the (usually bad) options, only for choosing ill amongst them.
If she has faced a genuine dilemma, she cannot be accused for facing it. If there were other options, they must be argued for; and, if there are none other than poor choices to make, that must be argued for as well. Blame is never enough, and neither is conjuring forks, tridents, bulls and crossroads out of thin air.
Further reading on fallacies: Madsen Pirie, How to Win Every Argument, the use and abuse of logic, Continuum, London, 2006. Fun, short, and acerbic introduction on the subject.