Suppose Joe makes the following argument:
- Every Lego in my Lego building is red.
∴ 2. My Lego building is red.
It's all too easy to accuse Joe of committing the fallacy of composition, which says that it's not generally the case that if a part of a whole has a certain feature then the whole itself has that feature.
A more charitable interpretation is that Joe's argument employs a hidden but reasonable premise, something like:
0. For any x and any y, if x is a Lego building, and y is a Lego piece, and x is composed of y, and y is red, then x is red.
Given a straightforward formalization of 0, 1 and 2, 2 would be a consequence of 1 and 2, and so Joe's argument would be valid. As such, Joe would be guilty of making a tacit but reasonable assumption. He would not be guilty of any fallacy.
All this to say is that it may be that you've committed no fallacy, but rather that your friend disagrees with a crucial premise in your argument and that because, without this premise, your argument would be guilty of the slippery slope fallacy, it's easier for your friend to accuse you of committing a fallacy than it is for him or her to confront your premises.
Perhaps we should call such a move "the Fallacy Fallacy"? What do you think? =D