When a person is interested in converting, there are really two questions they are asked:
- Are you prepared to accept the theology and observance of Judaism?
- Are you prepared to accept your identity as a member of the Jewish people, even in their suffering?
The Rambam writes that a person could keep kosher, learn the Talmud, etc., but if he doesn't feel connected to the Jewish people and doesn't care what happens to it, "such a person has no share in the G-d of Israel."
We've all heard news about the convert who had zero intention of accepting Jewish theology and/or observance; if it's completely clear that this intent was a joke (even with regards to their limited understanding of Judaism), then no conversion took place.
Has there ever been a case where the convert kept kosher, shabbos, and the like, but it was completely clear from the get-go that they had no intent of sympathizing with the Jewish people in any way? (I'm trying to think of an extreme example here ... pardon my politics and you're welcome to disagree, but maybe someone who goes right after their conversion to meet with Ahmedinijad at a Holocaust-denial event or something?)