What is blocking fault in volleyball?

When the blocker touches the net while blocking (called, "in the net"). Some think that a backrow player can come and block, but this NEVER happens unless you are in elementary school playing at a local facility for fun. The reff will notice this and call it, since the reff must know where the setter is at all times for both teams he knows where the players play and if they should be in backrow or not. Players will always notice it too.