When Frank tells Liz she is "really, really beautiful" her hair hangs differently between several shots of her face.
When Frank folds the card and tucks it into the pack of cigarettes the pack is about ¾ full. When the doctor finds the pack on the floor it is full. When Liz takes the card out of the pack it is no longer a tight, full pack. And when she throws it down it sounds empty or at least near empty.
He crashes the tanker truck in a ditch but it is then lying across the highway.
When Frank calls the Rhombus #, it's a standard US ringtone but the country code dialed (0041) is in England which has a different ringtone.
When Frank shoots across the street with a rifle everyone hears the shot, but when his partner did the same thing a few minutes earlier, killing Chloe, no one heard it.
When the President exits his limousine in the town the car door is an ordinary door not the thick bullet/bomb proof door of the Beast that he travels in.