Is it possible to believe (incorrectly) that one is on the path? Can one work one's way through the whole path as it is described in the Mahayana literature, and then realize: Even though you were being sincere, and the "right" experiences in the literature describe your work, you were actually not on the Buddhist path at all, but merely on an intellectual path without genuine actualization; without "direct seeing".

Can this nevertheless be preparation for the true Mahayana path?
Isn't there a story in the commentaries about a monk who had very advanced intellectual understanding of the Buddhist path, and was even a very good teacher of others, and brought many of them to enlightenment, but was not himself enlightened. The story goes that eventually something happened to make that problem clear to him, then he did some stuff -- lived in a cave and meditated or something -- and finally was enlightened. Maybe someone can remember who the guy was; IIRC it's a pretty mainline Buddhist story.

The suggestion is that, yes, it's absolutely possible to seem to be very advanced on the path but to be far less so than one thinks. And the second suggestion I take from the story is that if the intention is good, then even if one isn't as advanced as one might think, one may still be better off than having ignored the path completely.