Mary's father was hilarious. Be it his adamant story-telling or his complete oblivion while everyone else was very serious, something about him was just hilarious.
As freaky as it is, the chicken dinner scene is so absurd and funny that it must be seen to be believed.
Really, much of the movie can be this due to Black Comedy and Mood Whiplash present throughout, when it's not horrific. Another good example is when Mary is storming out, she walks out the door only to come back in and get her suitcase, which is stuck under the bed, which Henry is still laying on. She takes an awkwardly long time to pull the case out, while crying, and you aren't shown what she's grabbing until she gets it and leaves, leaving Henry's confused expression to pretty much emulate the viewer's.
At the very beginning, how slooooooow the elevator door closes. Anyone who has ever ridden in a slow elevator can appreciate this.