An unknown prankster sent in the plot of a two-part Seinfeld episode to the advice columnist Amy Dickenson. The episode in question is called "The Boyfriend" and just as the prankster writes, it deals with Jerry meeting his sports idol who then goes on to date Elaine.
Amy answered as if it was any other question with a short paragraph that offers solid advice and would have probably cut the end of the episode very short.
She admits on her own website that she missed the joke but she seems pretty good humored about it:
Every once in awhile, I get punked by a villainous, fun-loving reader. I have some affection for these episodes because I often think that if I didn’t actually have a job, punking advice columnists is exactly what I would do with my time. But alas I do have a job and so I am left with the task of sharing my humiliation, as well as trying to enjoy readers’ reactions.
Here's the beginning of the episode in question: