As Jennifer, Stefanie Powers keeps her fingernails rather short. The finger that presses the elevator button, the nail is considerably longer.
From the exterior of Jonathan's office building, there are no round windows, yet the window behind Jonathan's desk is a large round picture window.
The bad guy assaults the building guard and uses the guard's uniform as a disguise. Instead of it being two times too large, the shirt is actually too small.
The four people on the beach buggies in the chase scene are obviously doubles.
Marcie takes a red capsule. It's the same red capsule used in Death Set (1980)
FBI field workers are agents, not inspectors.
The bad guys watch Jonathan and Marcie leave from the building's garage exit, then somehow know that Jennifer, who they believe is Marcie, will be leaving the same building from the main door at the front of the building.