At the beginning, as the car speeds away from the manor, the woman has her window down and is waving to a little girl. As the car goes out of control, during the weaving you can see the window is still down, but when the car crashes into the lake, the woman is trapped in the car because her window is rolled up.
When Barnaby opens the deceased Doctor Frost's 1975 diary, the page for Monday 3rd January can be seen (as well as Wednesday 5th January) . However, the 3rd January 1975 was a Friday, and the 5th was a Sunday.
The piles of newspapers are of a uniform white color. However if they represent about 36 years of history, many of them would have turned yellow or brown by now as the paper pigmentation changes. Also some of the piles are too high to have been realistically assembled by a ladder leaning against them.