When Banacek locks Vickie in the trunk, he closes the center latch, but never closes either of the two side latches. When Vickie opens the trunk, she unlocks the center latch and opens both side latches as well.
When Banacek first enters the Merrick residence, he clasps his hands before him. When the scene cuts to a different camera, his hands are clasped behind him.
When Banacek leaves the Merrick house with Vicki, his movements of the steering don't correspond the background scenery. It especially noticeable when he goes around a corner.
After meeting with Maxwell Barker, there is a fade out/in to Banacek investigating the theater and equipment where Bradley Merrick disappeared. When he and Jay go into the dressing room, Banacek turns the light on - but it lights up before he turns the switch knob. Also, it is quite possible from the way it was done that he did not even turn the knob, but just faked it. The light is obviously controlled by someone else.
When Banacek and Vicki Merrick have dinner, she refers to the Peter Marshall character as Hal Porter. ("I like Hal Porter, but..."). The credits list him as Hal Parker, and that's the name Louise Merrick (Nancy Olson) uses when introducing him to Banacek.
When Vicki pulled the trunk out from behind the curtain she should have notice her father's weight was missing while she pulled the box.