If Player A hits the ball in the middle of the court & Player B has ample time to play the shot but sends it out or into the net, then it's an Unforced Error. However, if Player B is on the run while playing the shot & sends it out or into the net then it would not count as an Unforced Error but rather Forced Error.

What determines an unforced error in tennis?

A player marginally leans to the left in the middle of a point, and the opponent catches it in his or her peripheral vision and changes a shot at the last second seeking a strategic advantage. That is labeled an unforced error.

What makes an unforced error?

Quick definitions. Unforced Error: Missing a shot was the fault of the player who played the shot. Forced Error: Missing a shot was caused by the opponent FORCING their opposition to miss the shot.