Arc Fatigue: Krystle's kidnapping storyline in season 6 spanned over 13 episodes (nearly half the season), with increasingly frustrating results.
Ass Pull: The "Moldavian Massacre" conclusion, which revealed that the only major casualties of the massacre were Steven's lover Luke and Lady Ashley Mitchell, both played by guest actors. It's not surprising that the show steadily dropped in ratings for the rest of its run after this, and it would have probably gone down as the most preposterous plot twist in U.S. soap opera history had it not been for the All Just a Dream storyline in Dallas.
Bizarro Episode: Not from the series itself but its spin off, the final shot of The Colbys featured Fallon stranded in the desert, encountering a UFO, and being abducted by aliens. Her return to the series proper in season 8 featured her dealing with the fallout of this.
Germans Love David Hasselhoff: In the late 1980s this was the most popular TV series in Yugoslavia. It was broadcast every Monday at 9pm, and the streets were empty.
Growing the Beard: Season 1 had middling ratings, and not enough bite to sustain a major audience. Come season 2, more experienced writers, not to mention the introduction of Joan Collins, saw the show's ratings increase.
Harsher in Hindsight: Adam's numerous homophobic comments towards Steven and others became even more cringe-inducing when Gordon Thomson came out of the closet in 2017, and frankly discussed the self-loathing he'd suffered from being a gay man in his generation.
Never Live It Down: The Moldavian Massacre. Much like the All Just a Dream storyline with Bobby Ewing, it undermined the show's credibility quite a bit and it among one of the first things the average fan remembers about the show.
Replacement Scrappy: All of the second actors to portray the adult Carrington children are this with the exception of Steven. Most infamously are Fallon and Amanda, with the former suffering from Chickification (and making the list of "Television's 100 Dumbest Moments") and the latter was so unpopular, that it caused her character to be retconned out of existence.
Seasonal Rot: After the "Moldavian Massacre", there was no way that the show could top itself in terms of cliffhangers, and new storylines. The show's declining ratings reflected the show's rot.
Claudia Blaisdel-Carrington in spades. Becoming a mom at 16, getting married because of it, suffering a major mental breakdown that sent her to a mental institution for a year and a half, her failed reconciliation attempts with husband Matthew, getting involved with the gay Steven, Matthew kidnapping her daughter Lindsey and spiriting her away to South America, later learning that both were killed in a car wreck in Peru ( actually, Matthew survived, though Lindsey succumbed to her wounds), spent another year in the loony bin for another mental breakdown, got married to Steven though they later divorced, got involved (and married to) Adam though that relationship failed as well, and finally, her death in a hotel fire, unintentionally set by herself. All the while feeling that she was alone in the world.