Full Idea
We know that being fallible and being a priori can co-exist - the results of long numerical additions are well-known examples.
Gist of Idea
Long arithmetic calculations show the a priori can be fallible
Source
Frank Jackson (From Metaphysics to Ethics [1998], Ch.2)
Book Reference
Jackson,Frank: 'From Metaphysics to Ethics' [OUP 2000], p.47
A Reaction
I see this realisation as a good example of progress in philosophy. Russell, who says self-evidence comes in degrees, deserves major credit. It is the key idea that once again makes rationalism respectable.