I'd think the only way that would happen would be if the fresh cut post was stuck immediately into a moist hole so as to keep any "wounds" from drying out completely. 

I've seen a few "sprigs" pop out on freshly cut -- then immediately used posts, but those sprigs never got much longer than 4-6" and then the residual energy in the post was used up and/or the post hole dried out enough to 'kill' the growth.

The chances of all conditions being met to allow the sprigs to grow long enough to actually see the post "live", are very slim.  And if a few did sprout and live long enough to make you think they are going to "grow roots", spray them with brush killer.