I had the normal route in the fmc and was about 1500' deviation on path - then created long magenta on faf and put in intercept course because doing a ndb app.
Suddenly path showed about 2500' in the other direction.
Linecheck capt then was so kind to ask me why the path suddenly changed and were the new origin of the path was calculated.
Caus I didnt know the answer he told me in a kind tone to know this by the next time I saw him again.