Bohghossian and Peacocke write:
A priori justification is not infallible justification. Just as one may be justified in believing an ordinary empirical proposition that is empirically revealed on empirical grounds to be false, so one may be justified (non-conclusively) in believing an a priori proposition that is subsequently revealed on a priori grounds to be false.
I find this passage puzzling, for at least two reasons. First, Boghossian and Peacocke characterize a priori propositions for Boghossian and Peacocke as those which can be known a priori; so the idea of an a priori proposition that turns out to be false looks to me to be incoherent.
Second, it's not clear what the second sentence has to do with the first. The second sentence is about what may happen when you're justified in believing something -- that thing may turn out, either empirically or a priori, to be fase. The first sentence, however, isn't a claim about all justification; it's a claim about a priori justification. It can't be that a priori justification is fallible merely because it's possible to be justified in believing some a priori proposition that turns out false; if a priori justification is fallible, then there has to be a sense in which you can be wrong even if you're a priori justified. And that just isn't established or claimed in this passage. Is the idea supposed to be that any time you are justified in believing some a priori proposition, you're justified a priori? That would fill out the enthymeme, but it has the disadvantage to being totally implausible.
So I don't really know what Boghossian and Peacocke are up to here. Or, in general, what people who talk about a priori justification being fallible are up to.