So I was watching TV earlier today, and I saw a commercial for Scrubs on Comedy Central, and it had a scene where JD was commenting on a woman's nose, saying, "That is the worst broken nose I've ever seen," to which the woman responds, "I'm here because I stepped on a nail!" and I've seen that episode a few times before and it only now hit me what the joke was.

You can figure out the topic, go.

(Just to avoid the smartasses, it's times you didn't pick up on why a joke was funny until much later)