In the first episode of Bleach, Ichigo stabs himself in the chest with Rukia's sword to transfer her Shinigami abilities to him. Why does this not hurt him, while people can be hurt by hollows and while he is hurt by zanpakuto later in the show?
A zanpakuto is born out of a Shinigami's soul, and it obeys its master's will. When Rukia stabbed Ichigo with her zanpakuto, it was her will to transfer her powers to Ichigo, and hence Ichigo did not get hurt. If she had instead attacked Ichigo as an enemy, it would have hurt him.
Although this is not canonical, during the zanpakuto rebellion filler arc:
Ichigo did get hurt when Sode no Shirayuki attacked him.