In the teaser, Darrin opens the closet door twice before he goes to work.
When Endora and Sam appear in front of the office building, they are half-faded (to show they're invisible) but the entire shot was done that way, too. The pedestrians are half-faded, too.
When Samantha and Endora are flying to the alternate universe to see what became of Darrin, the wires holding up Elizabeth Montgomery are clearly visible.
Outside the "McMann and Tate" building (supposedly based in Manhattan, NY), you catch a brief glimpse of the San Gabriel Mountains in L.A. County.