Water has a mirror plane that bisects the H-O-H angle.  If the plane is thought of as a mirror, then one H is a reflection of the other.  Water has a second mirror plane (not shown) that includes all three atoms.

     Chloroform has a mirror plane that is that is defined by the C, H and one of the Cl's.  The two remaining Cl's are then reflections of one another.  In fact, HCCl₃ has three identical mirror planes, one for each of the chlorines.

     Square planer PtCl₄^2- has five mirror planes, two identical planes that bisect Cl-Pt-Cl angles, two that include a linear pair of Cl-Pt-Cl bonds, and one (not shown) that includes all 5 atoms.  In the former, Cl^a and Cl^b are reflections of one another, and Cl^c and Cl^d are reflections.  In the second, Cl^a and Cl^c are reflections of one another.