Twilight learned a new trick, by simply following her mentor's example. What Celestia, in her infinite wisdom, didn't realize is that showing her most faithful student such "tricks" could have lasting effects should she choose to replicate them.
Careful steps must be taken to ensure Twilight doesn't stumble down the same path the Once-King Sombra took to power.
Might be a little hard when he's the one teaching her.