When Mr. Drummond opens the curtains in the boys bedroom, you see a wall rather than a window.
Arnold and Willis's father is revealed to have been a Janitor working at the United Nations.
Irene Tedrow plays Phillip Drummond's (Conrad Bain) mother. In reality, Tedrow was only 16 years older than Bain.
It's very obvious that when Arnold, Willis and Mrs Drummond have a cup of tea together, there is clearly nothing in the cups.
When Arnold 'spills his tea' on Mrs Drummond, clearly nothing spills.
The comic book Arnold is reading at the beginning is Superman (vol 1) #331.