Mary and Rhoda write humorous obituaries late at night, unaware that one will be read on air the next day.
Lots of episodes of "The Mary Tyler Moore Show" thrust Mary Richards into situations where she must make up for circumstances out of her control, often only partly successfully. The reason this installment stands out in my mind is that here, Mary herself makes the unwitting but serious error and must react to its consequences.
The humor is supplied by the obituaries themselves, Ted's straight-faced on-air reading, and Mary's initial embarassment. What is more interesting is Lou - unable to accept the mistake (and telling tales from his newspaper days) - and Mary being initially contrite. The situation becomes more complex as she finds a level of uneasy pride and decides to make a stand, leading to a war of egos between her and her boss.
Situations like this are common in the workplace, and this episode does a decent job of transferring it to the realm of situation comedy.