After Maggie hit him with a mallet Homer said that his baby beat him up. In Eeny Teeny Maya, Moe he does get beaten up by a baby and is rescued by Maggie.
Homer and Marge take Maggie's attacks on Homer as being solely influenced by violent cartoons, but as we'll see in Who Shot Mr. Burns?, we'll see that Maggie really is capable of premeditated violence.
During the sequence of Marge's letter to the various people responsible for airing Itchy & Scratchy, Krusty the Clown is shown reading it and then flipping the paper around horizontally in an attempt to read it, alluding to his illiteracy, which had earlier been shown in Krusty Gets Busted.
When Roger Meyers Jr. sent Marge a rude letter in response to her complaint, it showed Meyers writing "So let me close by saying—" and then it immediately cutting to Marge reading the letter and saying "...And the horse I rode in on?". This was a reference to a very vulgar expression, known as "[censored] you, and the horse you rode in on."