In the scene where Ackroyd's butler, Parker, is drunk and staggering down the road, the car behind him stops. Visible for a brief instant is the car's license plate, COU 313. In the very next scene as the car begins its run, the license plate has changed to JHX 473.
The handwriting in the journal is not consistent from the beginning to the end of the film.
When Poirot and Japp leave the office near the end of the film, Japp is first down the stairs. However in the next shot it is Poirot who gets to the bottom of the stairs first with Japp following.
A wedding ring has been found with the initial 'R'. It is suggested that 'R' might be the secretary Geoffrey Raymond. But no-one would inscribe a wedding ring, to their lover, with the initial of their surname.
In the journal, the word 'occasion' is misspelled as 'occassion'.
At the airport a modern day Cessna can be seen taking off.
Many of the characters call each other by Christian names - Poirot calls Sheppard 'James', Caroline calls Poirot 'Hercule', Raymond calls Ackroyd 'Roger', though not to his face. This would never have happened among the upper-middle classes 100 years ago - Christian names were reserved for lower-grade servants and children. Poirot would have called Sheppard 'Dr Sheppard' or 'Sheppard', and so on. Naming conventions were carefully calibrated to reflect class distinctions.
During a slow right to left tracking shot of Poirot in a Blue Taxi, a very modern, black 2-way radio is lying on the running board of the car under the driver's left hand.
At the beginning, Poirot writes all kinds of disparaging things about Roger Ackroyd giving the impression that he barely knew him and disliked him. When he meets him the first time there is no acknowledgment of friendship, in fact there meeting is quite cold and businesslike. However after the murder later on he claims to be his old and very dear friend, a clear contradiction of what happened earlier.