
Major Offenders: 24, Alias, Nip/Tuck, Star Trek: TNG, many others.
This is when a show teases us with a cliffhanger, followed by an episode that returns everything to normal within minutes.
So, in 24 Jack Bauer winds up in a Chinese prison at the season finale. How will he possibly get out of this one? Oh, wait, he just walks off a plane in the next season opener, back in the good ol' USA and with a kick-ass beard for his trouble.
In one season finale of Star Trek: TNG, Commander Riker has to make the terrible decision to destroy the bad guys' ship with a captured Captain Picard still on board, ending the season with his pivotal decision to "Fire." We wait for the next season and, wouldn't you know, the weapon has no affect. Nevermind!

We can thank the 80s drama Dallas for starting this. They cashed in with the biggest cliffhanger of all time, when villain J.R. Ewing got shot in a March 1980 episode. After a long summer break (when "Who Shot J.R." became an international catchphrase) it was revealed that J.R. was alive, his would-be assassin was let go without any criminal charges, and the whole thing was barely spoken of again.
Dallas, determined to top this retarded publicity stunt, years later opened a season by declaring everything that happened in the season before it was a dream.

"Surprise!"
Why it Works:
Production schedules force most shows off the air for months, up to a year in some cases. The problem has always been that fans can wander off during the down time, so cliffhangers keep people talking through the dry months (in the case of "Who shot J.R.," the next episode got a then-record 83 million people to watch).
And, once the show comes back, who cares that we bailed out of the cliffhanger with an unsatisfying resolution? You should just be glad the show is back at all, you ungrateful fuckers!
Why it Shouldn't:
It's in these cop-outs that a cliffhanger is revealed to be purely a marketing gimmick, having no actual impact on the storyline. These cop-outs let the writers off too easy, since they get to put the character through some kind of life-changing trauma, then just have them get over it (Jack Bauer recovers from his lengthy Chinese imprisonment just a few hours into the new "day.")

Where's the crippling depression that leads to alcoholism, or the post traumatic stress and years of counseling? They turn our surviving heroes into heartless bastards who don't care about anyone or anything for longer than a 2-hour season premiere.
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Couples That Constantly Break Up and Reunite

Major Offenders: The Office, Friends, Sex and the City, Scrubs, countless Soap Operas.
After months or years of increasing sexual tension, two leads finally admit that they love each other and want to be together. This usually occurs with a passionate kiss and a high pitched "Whoooo" from the studio audience. We at home get to believe that we, too, will one day find true love with the one hot girl in our circle of friends.