Dry-Docked Ship

This trope is about characters who wouldn't fit together now, for whatever reason, but about whom there is a general consensus in the fandom - even in fics focused on another pairing - that they used to date/have slept together at some point in the past.

This does not apply to Canon exes.

May be secondary to another pairing, not considered a threat but treated rather condescendingly, as in the Merlin examples below. Alternatively, it may be a popular ship which assumes that the Foe Yay they have now is due to a horrible, horrible break-up.

Compare Starboarding.

Anime and Manga

 * In Fushigi Yuugi slash fic revolving around either Tasuki or Kouji, it's very often assumed they're either former lovers or friends-with-benefits, regardless of whatever the central pairing of the fic turns out to be. (Everyone Is Gay, after all.) Canon more strongly seems to indicate they're Heterosexual Life Partners.
 * Also, Hotohori x Nuriko
 * In Spiral, almost any fic featuring slash and dealing with Kanone and/or Eyes, but not being primarily KanonexEyes, will still mention them at least having been together at some point. Kanone is very, very often the psycho and/or dead ex when the author wants to pair Eyes with another guy.
 * This is a common occurrence in the Axis Powers Hetalia fandom, thanks to the characters being nations who presumably have had numerous alliances, treaties, and flucutating relations with other nations over the course of history. For example, while America/Lithuania isn't a hugely popular ship, even some fans that ship America with someone else think that the two probably had a thing going on during the twenties when Lithuania was living at America's house. Similar cases have also been made for England and France, America and Japan, England and Japan and Russia and China. And of course, even the most ardent Prussia/Hungary shippers have to acknowledge that Hungary used to be married to Austria.
 * Don't forget Russia/Finland and Sweden/Norway.
 * Sometimes after Sakura Kyoko made her first appearance in Puella Magi Madoka Magica, people suspect her of having had intense relationship with Tomoe Mami in the past, since she did acknowledge knowing her. And then canon materials were released to flood the dock: they did have a relationship, but the situation in Kyoko's house made her adopt a paradigm that goes completely the opposite of Mami's.

Comic Books

 * X-Men - Magneto/Professor X. Magneto's actor in the movie, who happens to be gay, has made allusions to playing the character as such.

Film

 * GoldenEye: The way James Bond reacts to Alec Trevelyan's betrayal seems more like a lover's reaction than a co-worker's. Cue Foe Yay.

Literature

 * Harry Potter: Dumbledore/Grindlewald, or at least Dumbledore -> Grindelwald has been confirmed by Word of Gay.
 * Even before, most Remus/Sirius shipping was set back when the Marauders were still at Hogwarts.

Live-Action TV

 * Heroes has Claude/Bennet, whose fics tend to be set before Bennet tried to kill Claude.
 * Giles/Ethan as well as all manner of vampire shipping in Buffy the Vampire Slayer.
 * Spike/Angel has been just shy of confirmed by Word of God.
 * That one time...
 * It has been very much confirmed by Word of God on the commentary for the Angel episode "A Hole in the World."
 * Merlin/Will, Merlin and/or Arthur/Lancelot, and past, unrequited or simply unconsumated Uther/Gaius and/or Uther/Nimue.
 * Buddies Helo and Kara of Battlestar Galactica are widely regarded to have slept together prior to the series.
 * Over on The Mentalist, most people think that Bosco and Lisbon slept together at least once. Lisbon has moved on. Bosco hasn't.
 * There is some debate on whether or not Robin and Marian slept together pre-show, prior to Robin leaving for the Crusades. There's no consensus, but fics that assume or confirm that their relationship was consummated are not considered out of the question (even for those that ship another couple). There is also some evidence in the show itself, particularly the deliberate avoidance of the usual "Maid" when referring to Marian. The creators have stated that they went with "Lady Marian" because they wanted to suggest that Robin and Marian had a history together. Of course, there were also several innuendoes in the show that suggested Marian was still a virgin, so the question still remains.
 * There is speculation in the Firefly fandom that Kaylee and Jayne hooked up a couple times in the past, in a friends-with-benefits sort of formula.

Webcomics

 * Alexander/Beatrice in The Dreamer. There are several hints within the first pages of Alexander's introduction that can lead to this conclusion, such as Beatrice and Alexander living together for an entire summer and also this little quote: "Ah yes, I remember taking my turn more than once with you that evening".
 * Gamzee/Tavros in Homestuck was only canonised long after one half of the pairing went crazy and the other half died, and then left rather ambiguous with no Word of God on the topic at all (it's obvious Gamzee is carrying a torch for Tavros, but whether or not it is requited is not explored). If you believe they ever had a relationship, it automatically becomes dry-docked by the time it is relevant in the story, when Gamzee is rather desperately making out with Tavros's severed head.

Western Animation

 * In My Little Pony Friendship Is Magic, many fans believe that Celestia and Discord are exes.
 * As well as Rainbow Dash and Gilda.
 * Same thing to Marceline and Princess Bubblegum from Adventure Time.