'Jill' is sort of a moocher! ** POSSIBLE SPOILERS **
Clearly, neither of the characters have much money. But every time a meal is served, it's always on Don's dime. Then when Don's mother is present and it's pointed out that there's only lettuce and an apple in the fridge, Jill lays claim to the apple. Then Don's mother invites Jill out to lunch. (I'm sure Jill doesn't offer to contribute) Then in the 'dinner on the beach' scene, Don seems to eat nothing, whereby after about two minutes, Jill claims she's just eaten the last of the salami. Then in the final act, he say's he's just about to make a corned beef sandwich for himself. She agrees to have one too...but he makes only ONE sandwich, gives it to her, and then she never touches it.
Maybe it's nothing strange...but just a common theme I noticed.