Gwendolyn describes three cars in a row as "green, white, and red, in that order", going on to say that "one thought inevitably of the flag of the Irish Republic". Ireland has never used that color combination in their flag. Gwendolyn's thinking of Italy.
Ernesta at one point notes her sister Gwendolyn hasn't missed an issue of Photoplay since it started in 1923. It actually began in 1911.
When Barney is asked to turn off the lights for the film screening at the unmasking of the murderer, the lights dim before his hand reaches the switch, and the audible click of the switch has no relationship to the fading of the lights on the set.
The 16mm film cans that The Snoop Sisters retrieve from a film lab and bring to their nephew are clearly empty.
When a red car in hot pursuit drives up into a car carrier trailer and stops - the shadow of the camera and cameraman can clearly be seen.
During the final chase sequence, the sisters manage to get from Forest Hills (71st Avenue, just south of Queens Boulevard) to Court Square in Long Island City - a distance of over five miles - in a matter of seconds.
The murder victim wears white. A gun is involved. The death is mildly redressed to look like suicide. There is no blood anywhere. Not a drop.
At a certain point, the projected film stops and the room lights come up simultaneously with a nod of Lt. Ostrowski's head. That's too tight. There should have been some lag time; plus, Barney shut off the lights then sat down, but when the lights come up, Barney is still seated with the guests.