It doesn’t make sense, really, yet it’s a truism: Creators of classic television shows shouldn’t write episodes of the series once they become classic. This holds especially true with a series finale. The occasion usually prompts a creator to try too hard and overdramatize, missing the subtle dimensions of past episodes written by others. A typical example was the disappointing return of co-creator Larry David to “Seinfeld” in May 1998 to write that show’s last episode; it’s hard to forgive his disastrously absurd two-parter, in which the foursome lands in jail after violating the Good Samaritan law of a small Massachusetts town. Another blown opportunity came with “An American Girl in Paris,” Michael Patrick King’s pretentious two-part 2004 “Sex and the City” conclusion, which was cloyingly emotional where it should have been achingly funny.

Last night, sadly, came the latest installment in this chronicle of failed finales: Alan Ball’s 75-minute conclusion to “Six Feet Under,” the astonishing HBO family drama he created in 2001 and nurtured through five seasons of gently rendered subtext, at least most of the time. The Fishers lost their patriarch in the pilot episode, and his frequent guest shots as a ghost over the next five years set the tone for this heart-wrenching roadmap of family wounds. Setting the show in a Los Angeles funeral home, Mr. Ball picked up the themes of dysfunction he’d addressed with such poetry in his Oscar-winning screenplay for “American Beauty.” The (barely at times) surviving Fishers seesawed between joy and pain with almost reckless speed; they were a joy to watch, this maddening family of death maidens with their hearses and their heartbreak.

The last few episodes hurtled by. Written by Mr. Ball’s trusted lieutenants (the distinguished list of “Six Feet Under” writer-producers includes Craig Wright, Kate Robin, and Bruce Eric Kaplan), recent developments like Nate’s adultery and death, David’s descent into madness, and Claire’s dalliance with a Republican had kept the show humming. Nate’s collapse after sex with Maggie (yes, the famous “Narm!” moment) couldn’t have been more shocking. Yet it didn’t quite prepare audiences for the shock of Nate’s eventual death. It took guts for Mr. Ball to resist that as a final episode gambit, and move it instead into the heart of the season – and it worked spectacularly.

Until last night.

It was for the series finale that Alan Ball returned to write and direct, and once again a creator made the same mistake. The episode Mr. Ball delivered last night on HBO was everything “Six Feet Under” had kept itself from being for so long and so well. Everyone cried; everyone said what they were thinking; everybody made choices; everyone figured out what they wanted; and then, at the very end, everyone died. It was as though Mr. Ball had set out to find the most cloying conclusion possible, and opted for every available cliche. Yet for all its weaknesses, no one will allow the failure of the finale to color their view of a show that still deserves its status as a classic; like the Fishers, we must learn how to forgive.

There were moments of searing emotional power last night. When Ruth remembered that her surviving son, David, liked his cereal in a yellow bowl, it captured with one vivid line – “Yes, Mom,” David said, “put it in the yellow bowl” – the powerful bond that connected the remaining Fishers as they mourned their fallen family members. The transformation of Keith and David into parents was presented with special poignancy, and when Mathew St. Patrick (the wonderful actor who has made Keith Charles such an integral part of the show) raised his glass, “To Nate,” at the farewell party for Claire, he began a scene of eventually excessive emotion with remarkable restraint.

It was unfortunate to watch Frances Conroy being forced into so much public grief; her sublime subtlety and suppressed emotions as Ruth Fisher made her the pivotal figure for most of the show’s run. (She was the 21st-century equivalent of Miss Ellie on “Dallas.”) As David Fisher, Michael C. Hall chewed enough scenery in the finale to give himself some serious digestive problems; he cried his way through virtually every scene. The script also asked too much of Lauren Ambrose, whose performance as Claire Fisher in recent weeks has been especially hilarious and honest. Mr. Ball had given Claire some magical bits to play early in the finale – especially in her deepening relationship with Ted – but then directed her into a last-minute reckoning with her mother that felt forced, if not false.

The finale presented Claire’s decision to move to New York City as the defining moment of the series; as she heads out on the highway (with “Ted’s Deeply Unhip Dance Mix” on the car stereo), we’re treated to Mr. Ball’s vision of all his characters’ deaths in the future, and of their physical appearance at their last moment on earth. As much as we might generously want to look at this exercise as a well-intentioned mistake, it discolored the experience of watching a great show’s final episode. The sight of Claire, David, and Keith as elderly and near death didn’t advance our understanding of them; it diminished them into stock figures on a second-rate daytime soap.

Which, thanks to the great gifts of Alan Ball, these characters have never been. It’s hard to imagine a better family drama than “Six Feet Under” on television anytime soon, or a writer more singularly responsible for its success – except, perhaps, the even more astonishing David Chase, creator of HBO’s “The Sopranos.” Let’s hope Mr. Chase delegates the task of writing the series finale of “The Sopranos” to others.