The UK Series
- Casting Gag: Or, rather, guest performer gag. Amber Riley auditioned for American Idol and was rejected by the producers and ended up on Glee instead. And then the Glee cast, including Amber, performed on the semi-finals results show of series 8. Amber sang her big solo right in Simon's face.
- Colbert Bump:
- Being the show's biggest success story, One Direction served as the gateway for other X Factor alumni, such as Olly Murs, Cher Lloyd, and Little Mix, within the US market, and helped garner interest in the show's short-lived American version.
- If anyone's to blame for the sudden runaway success of "Human" by Rag'n'Bone Man in the UK (although it was already in many top tens in Europe by than), it's Emily Middlemas from series 13.
- Corpsing: The judges tend to do this during the audition rounds, especially with the really mind-bending bad ones. Part of the reason Cheryl was so popular on her first season was attributed to viewers liking the fact that she just couldn't keep a straight face. (She's gotten better at it, but she's usually still the first judge to break.)
- Andrea Faustini's performance of "Relight My Fire" left Simon unable to coherently critique him because he was laughing too hard at the fact that Andrea was completely painted gold, with devil horns.
- In an early season, Sharon and Louis suffered violently from this in front of one contestant. When the contestant told the judging panel that she was a horse jockey, the two of them spluttered with laughter throughout her entire audition. Outraged, Simon ordered them to leave because they were distracting the contestant as she tried to sing. As the two of them left giggling, Sharon smacked her head against the door, making them laugh even more.
- In series 12, poor Cheryl absolutely broke down crying
trying to maintain her composure during boot camp after one contestant threw confetti into the air during his audition.
- Creator Backlash: Simon largely disowned the show's first Celebrity Edition Battle of the Stars, calling it pointless and ruling out another series in an August 2006 interview for the third series. Eventually he would do another attempt in 2019 with Celebrity.
- Development Hell: The All-Stars edition (which was planned to debut after the celebrity edition with Paula Abdul becoming a judge for said edition) was postponed indefinitely in November 2019 due to lack of interest from previous contestants, resulting in Cowell to launch a band-focused edition for the 2020 (later December 2019) season titled The X Factor: The Band, a Spiritual Successor to the ITV show Popstars, which is best known for launching the careers of Girls Aloud.
- The Pete Best: Brian Friedman has the honour of being the shortest lasting permanent judge as his chemistry with the other judges during the London auditions in series 4 didn't work well, with Sharon finally realising that she missed Louis, given that he was not originally intended to return for said series after being relieved of his duties and replaced by Friedman. So as a result, Friedman was relegated to creative director and Louis was reinstated back.
- Promoted Fanboy: Nick Grimshaw has repeatedly made it known that he's a longtime fan of the show and frequently has the contestants and judges on his radio show before finally joining the show's 2015 season as a judge.
- Real Life Writes the Plot: Cheryl's absence in the last set of auditions and bootcamp for series 7 was due to falling ill from malaria.
- Recycled Set: The Cowell-produced "That Dog Can Dance" reality show shown during Christmas 2012 uses the X Factor set, albeit badly disguised.
- Role-Ending Misdemeanor: Tulisa was booted from the show for being caught participating in a drug smuggling ring. She was eventually tried and acquitted and has since spent her time trying to repair her image and still remains on good terms with Little Mix, who she mentored.
- In the 2016 season, the duo Brooks Way ended up axed from the live shows following claims that Josh Brooks had beaten up his ex-girlfriend.
- Romance on the Set: Ryan Lawrie and Emily Middlemas during the 2016 season, though it's downplayed in that they were already dating when they auditioned, but had to "fast-forward" the relationship upon moving into the X-Factor mansion after making the live shows. According to the tabloids, before Ryan's elimination, they couldn't keep their hands off each other and were constantly kissing and cuddling even during rehearsals.
- Cheryl wound up having a 2 year relationship with Liam Payne long after One Direction's run on the series, and they had a child together in 2017.
- What Could Have Been: When Simon Cowell departed for the US, who was his first choice for his replacement? Noel Gallagher. Simon himself offered the job but he turned it down which led Simon to go to Gary Barlow but just imagine what the show would've been like the bad boy Oasis front man as a judge...
- Sharon was going to be a live shows only judge in series 15 but withdrew. Had it happened, it would've been the first time that the show had five judges.
- Not only that but she was expected to return in series 5 but withdrew before auditions began due to her disliking of Dannii (this due to the rivalry between the two during the series 4 live shows) and was replaced by Cheryl.
- Nicole was not originally expected to replace Kelly for series 9 as it was originally planned that a new judge would be revealed during the bootcamp stage but due to positive feedback, her position became permanent.
- You Might Remember Me from...: Season 1 quarter-finalist Rowetta Satchell had a successful career in the 1990s indie dance scene, particularly as a member of the popular Madchester group Happy Mondays. The show defended her participation, telling the media that the show was open to anyone regardless of their skill level or professional experience.
- Season 11 runner-up Fleur East previously made it in the live shows in the second season as part of the girl group Addictiv Ladies.
- Saara Aalto from Season 13 has quite an extended curriculum; in addition to her stints on Finland's selection shows for the Eurovision Song Contest, where she placed second both times, she acted as the voice of Anna in the Finnish dub of Frozen (2013) and performed with Andrea Bocelli of all people.
- Also from Season 13 is Ryan Lawrie, who previously performed to a crowd of thousands at the 2016 Capital's Summertime Ball after winning the Vodafone Future Breakers competition.
- Slavko Kalezic in Series 14, an audition that was teased on the show's preview but saved to one of the last audition episodes. Slavko had been involved in the most recent Eurovision Song Contest (2017's) for the socially conservative Balkan resort of Montenegro, although he failed to get them into the final, despite plenty of fan admiration of his song "Space", which involved homoerotic lyrics, a gender fluid style, in which he wore a see through shear top, skirt and tights, which won him the unofficial Dex Prize for the show's worst dressed act, and his amazing 6 foot braid. He was chosen by the Montenegrin broadcaster to partake, avoiding the selection processes Saara Aalto lost for Finland. Slavko performed the Beyoncé song "End Of Time", which he also used as an audition song when performing on the Pan-Yugoslav series of the X Factor in 2013. It emerged Louis Walsh had invited Slavko to compete in X Factor when meeting him in Kiev during that Eurovision, whilst he was supporting his protégée Brendan Murray.
- And the ex-Hometown singer himself partook the following year, though with Walsh no longer a judge. Ironically for a singer whose failure to qualify was called "Dying to Try", he was allowed to change his song by Simon Cowell during his audition before being allowed through. Has since became one of the sensations of the series, going into the live shows, getting Louis Tomlinson's golden buzzer and making Nile Rodgers cry!
- There were 2 former members of the ESC domain in an audition episode later in series 15, although Romania's iconic Opera entrant Cesar, from the 2013 ESC, where his bizarre and homoerotic vampire-opera-dubstep won widespread intrigue, and an ex-collaborator of Bocelli, looked different 5 years later as he auditioned wearing a fedora and glasses. The voice still stood out as ever. The other one was a former winner of Kid's Eurovision later that same year, from Malta, although UK were not involved in that event between 2005, and Wales partaking alone 13 years later.
- Season 15 contestant Janice Robinson had been the vocalist on the 1995 club hit "Dreamer", which she sang at her audition.
The US Series
- What Could Have Been: One would wonder if the show became more successful had Cheryl remained as a judge than being controversially replaced by Nicole.
- You Might Remember Me from...: Averted in the first season. Although contestant LeRoy Bell cowrote two hits for Elton John in the late '70s ("Are You Ready for Love" and "Mama Can't Buy You Love"), this was never referenced during his run on the show, and he did not perform either of these two songs even for his "save me." Interestingly, two other contestants - ex-Disney star Christa Collins and Audrey Madison-Turner, former backup singer for Ike Turner (and his widow) - did have their past successes discussed on the show, but mentor Nicole Scherzinger eliminated Collins during the "judges' houses" phase, and Madison-Turner failed to make the bootcamp cut.
- Likewise, the previous career successes of fellow Over 30 contestant, Stacy Francis, such as being in a girl group "Ex-Girlfriend" and originating the role of Rusty in the musical version of Footloose on Broadway, were never addressed on the show. Of course, this could be because she had claimed in her original audition that she'd had no real success in show business and the truth made her seem like a less sympathetic character.
- Arin Ray in the second season, who had previously made it the live shows the year prior as part of the group InTENsity.
- Don Phillip in the second season audition episodes, who dueted with Britney on her debut album. Britney regretfully, but honestly voted against him.
- 13-year-old Carly Rose Sonenclar has some acting credits to her name and even orginated a role on broadway.
The Australian Series
- You Might Remember Me from...: Samantha Jade had moved to America for a recording contract before Executive Meddling stopped anything from coming of it. This is commented on in-series.
