When one character becomes so sidetracked by one train of thought that he completely forgets about the original topic.
Characters prone to this include the Cloud Cuckoo Lander and The Ditz. The Only Sane Man (who is often subject to this) can respond with a facepalm, some form of internal monologue expressing his disbelief, or an attempt to remind the invoker of this trope what they were talking about in the first place. Occasionally, the subject material brought up in the Derailed Train of Thought can result in the recipient receiving Too Much Information. Responses may include the Spit Take, and for added humor, the invoker of the Derailed Train of Thought may remain unaware of the reaction he/she has caused.
Sometimes, the invoker of this trope may not be a Cloud Cuckoo Lander, but is extremely embarrassed and, as such, wants to turn the subject to something different. Or it could be a Cloud Cuckoo Lander who also happens to be extremely embarrassed.
Compare also with Non Sequitur, in which it is heavily implied that the character should be discussing one thing, but one of them goes completely against those expectations. In this trope, it is made clear by an initial statement what the conversation is about, and then one character turns the topic to something completely different. Observe:
In Non Sequitur (taken from their page):
In Derailed Train of Thought:
Notice that in the second example, Alice specifically stated that they were going to talk about cell phones, whereas in the first, it was merely implied.
- This Series of Videos by Microsoft's Bing Search engine.
- Azumanga Daioh: Osaka, Cloud Cuckoo Lander extraordinaire, does this at least once, with a discussion on japanese characters that starts with hemorrhoids and goes on to the proper way to write kanji, and then to sea life.
- In the Bleach anime, Orihime manages to produce the epic Leekspin meme in an Epic Tangent resulting from a crash-in with Ichigo in their school hallway. He asks if she's okay, this results from her embarrassment of bumping into him.
- In Fullmetal Alchemist, Ed would rather recite the periodic table than talk about his potential love for Winry.
- Everyone who tries to talk to Isaac and Miria. Everyone.
- In Calvin and Hobbes, Calvin often goes off into these. Watterson, apparently, based Calvin's tangents off of his own wife.
- Happens briefly to Lightning McQueen at the beginning of Cars when pysching himself up for the race.
Lightning: I eat losers for breakfast... Should I have had breakfast?
- Played with in Lethal Weapon 4, where Chris Rock and Joe Pesci go off on a huge tangent about cell phones, for no apparent reason.
- In Inglourious Basterds, Colonel Hans Landa, in the middle of a conversation that will alter the course of history, pauses to ask his native English-speaking captives if he is properly using the expression "Bingo!"
- Harry Lockhart does this in Kiss Kiss Bang Bang, to much hilarity.
- In Carry On Matron, Sid Carter and his partners in crime, Freddy and Ernie, are planning to steal a stockpile of birth control pills from a maternity hospital. As they look over the plans for the hospital and Sid explains where the pills are kept, Ernie asks where the babies are kept. An impatient Sid tells him they're in the wards, of course, but Ernie says it's not quite so obvious - he was born on the top deck of a Number 73 bus in Brixton High Street. Freddy says that's impossible as the Number 73 goes to Putney and Wimbledon, not Brixton, and then Sid says they're both wrong, it goes to Holloway (where he used to visit his wife in the women's prison).
- Luna Lovegood of Harry Potter often manages to turn a conversation into something about the magical equivalent of Bigfoot and the Loch Ness Monster. Many of the conversations she starts begins with her saying something completely ordinary, and then turning it into this trope.
- Neal Stephenson's Anathem seems to do this several times, but every single one of them turns out to be a Chekhov's Gun of some sort.
- The narrator of Mark Twain's "The Celebrated Jumping Frog of Calaveras County" asks an old man about someone named Leonidas W. Smiley. The old man says he has never heard of anyone named Leonidas W. Smiley, but he has heard of someone named Jim Smiley. The rest of the story comprises a Rambling Old Man Monologue about Jim Smiley and his jumping frog.
- The Dowager Duchess of Denver tends to change the subject four or five times - in rapid succession - whenever she opens her mouth.
- In Dave Barry Slept Here, Thomas Jefferson, who is writing the Declaration of Independence in an all-nighter, lets the document's subject wander to people flushing inappropriate objects down toilets, among other things.
- One Polish poet once wrote a cycle of micro-plays and other stories. One of them - that long - includes such a verse: "Unaccustomed as I Am to Public Speaking..., let me say, that... (here happens a two-hour-long volcanic eruption of speech, which is completely unrelated to the topic)"
- Ciaphas Cain (Hero of the Imperium)'s stream-of-consciousness memoirs occasionally drift off topic and he brings himself back to the point with "But I digress."
- The ghost of Cyril Pennyfeather does this in his conversation with Lori and Dimity in Aunt Dimity Goes West. Cyril is explaining how he calmed Lori and Dimity while going unnoticed by either of them:
"Good grief," I said softly. "You made my nightmare go away."It would be more accurate to say that I created an atmosphere of tranquility and security in which you found it easier to sleep, and sleep, saith the Bard, is the balm of hurt minds, great nature's second course, chief nourisher in life's feast. Macbeth Act two, Scene two. But I digress.
- Tristram Shandy is made of this. The book is about the supposedly interesting life and opinions of Tristram Shandy, but the narrator keeps getting sidetracked, not even getting to Tristram's birth before partway into the second volume.
- Our Miss Brooks: Absent-minded Mrs. Davis often suffers from thought derailment.
- In Seinfeld, George's boss, Mr. Steinbrener, always gets sidetracked by one thing or another when George tries to talk to him.
- The Doctor, on Doctor Who.
This is my timey-wimey detector. It goes ding when there's stuff. Also, it can boil an egg at thirty paces... whether you want it to or not, actually, so I've learned to stay away from hens. It's not pretty when they blow.
- The Swedish comedy show Hipp Hipp had Melodikrysset, a musical crossword aired on radio, wherein the host would play a song and then explain whet you were supposed to write. After playing In the summertime the explanation is as follows.
The Host:That was In the summertime by Mumbo Jerry and it's about the summer, in the summer people like having ice-cream. Personally I'm fond of 88:an. 88:an has crushed nuts in it so therefore Pungspark goes on vertical no. 13.
- The Newsroom features this regularly, most notably when a meeting where a discussion regarding the possibility of American troops having committed a war crime segues into a long discussion about whether Santa Claus has eight or nine reindeer.
- The Monty Python's Flying Circus episode "Erizabeth L" has a police inspector (from the Film Fraud Division) who's apparently incapable of arresting a dangerous criminal who's impersonating famous directors without digressing into a lengthy biography of the director in question.
- Sophia did this practically every time she told a Sicily story on The Golden Girls. She'd get off on a tangent, then say "But I digress."
- In Outnumbered, a question to Pete from Ben about wher Friday the 13th beliefs originated ends with Ben describing how someone could end up accidentally crucify themselves.
Pete: Yeah, I think we may have gone on a bit of a tangent.
- The "Weird Al" Yankovic song "Albuquerque" is a long, rambling screed that starts off with an account of his mom serving him sauerkraut for breakfast that touches on the number of molecules in Leonard Nimoy's butt, moving out of the house, playful shriners, fluffy towels, ukulele-playing lepers, back-shavers, Albanian women with really bad body odor, Doctor Pepper, an airplane crash, an overly stuffed suitcase, eating soup out of ashtrays, air conditioning, those little mints that housekeepers leave on your pillow at high-class hotels, hermaphrodites with that Flock-of-Seagulls haircut, weasels, peaches, Zelda, mint-flavored dental floss, working at the Sizzler, excessive ear wax, Torso Boy, and a lucky autographed glow-in-the-dark snorkel, all to end up at the entire point of the song:
I know it's kinda been a roundabout way of saying it, but I guess the whole point I'm tryin' to make here is I HATE SAUERKRAUT!"
- Girl Genius: Count Wolkerstorfer has a habit of getting himself off track, an example from a section of his conversation with Dimo and Krosp when he was attacking the Corbettites:
Wolkerstorfer: You fools are all so smug and superior-treating my like a scatterwit! Well, I can and will destroy you all! ...and then I'll return to the king in triumph, and we'll see who-um.. er... Say, don't you hate it when you're in the middle of something, and then you completely forget what you're doing?
Dimo: Ho, yez.
Krosp: Um... You were just leaving?
Wolkerstorfer: No, no... Give me a minute... now what was I doing... um... Martellus is back... he's the Storm King... The Order's in chaos... like always. I developed a better magnet... Lord Whazizname's still mad at me because of that cheese thing... um... Lab destroyed... Was given ultimatum... yadda yadda... Killed that enforcer guy with my magnet-so score... Solved the Collatz Problem... mm... really should write that down...
- Many of Seth MacFarlane's works are prone to this. Family Guy and The Cleveland Show employ 10-second shorts to provide comedy, connected only to the dialogue by a "Just Like X" statement made by one of the characters.
Brian: ...That depends. Do you really want my advice or are you just asking random questions?Peter: What's a hypotenuse?
- Invader Zim - Don't even try to have a coherent conversation with Gir.