Harsher in Hindsight: A strip dating back to 1986 featuring Dennis tricking Walter into going to school dressed up like Rolf Harris, which is met with a stunned reaction from the class. It takes on a chilling note almost 30 years later after Harris was sent down for allegations of child abuse.
The earlier strips had Dennis' Dad almost continuously spanking him with a slipper, usually to no effect; the only thing that could make an impression was Granny's legendary "Demon Whacker" slipper made from the hide of an African Elephant. This was phased out as corporal punishment of children fell out of favour.
In one old strip, Dennis' Dad spanks him for falling in luminous paint, even though it was an accident. In another, Dennis leaves home, and his parents repeatedly appeal for him to come home, but when he does, it turns out they wanted him back because he had accidentally taken the Darts Club funds with him — there's no indication that they were worried about their only child (then he gets spanked for running away).
Notably, it's implied, by the Big Reveal of who the modern Dennis's father is, the reason his current father is more lenient is that he knew exactly how it felt...
Viewer Gender Confusion: The gender Foo Foo, Walter's pet poodle, seems inconsistent. It isn't just Gender Equals Breed expectations, Foo Foo also wears a pink ribbon, so many readers thought it was female, but the dog was usually referred to with male pronouns in the comics. However, in the animated TV series Foo Foo was referred to as female.