Please don't list this on a work's page as a trope. Examples can go on the work's YMMV tab.
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.
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 fluctuating 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.
Though generally not focused on much by cannon or fandom, Austria also used to be married to Spain and, it is suggested, France. Austria has been married to quite a lot of people actually. Apparently he really gets around.
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.
There've been a lot of serious hints dropped that Harvey and Donna on Suits were once in a romantic relationship.
As of episode 3.06, a one-night-stand has been confirmed.
The idea that KC and Bianca have slept together at least once seems to be one of the few fully agreed upon and common opinions in the DTC ("Degrassi" Tumblr Community)
There was an absurdly popular Glee fan theory for a while (back when a lot of viewers were holding out for Sam to be Bi the Way) that Sam had transferred from Dalton and he had dated Blaine while there. More recent developments (namely, Sam's poverty and continued lack of interest in guys) have put this to rest.
Many Doctor/Master fans assumed that the two went through a break-up of some kind back on Gallifrey, especially given that the Expanded Universe made it explicitly clear that Theta Sigma and Koschei note though this might be a nickname like the Doctor's, as the writer using it as his real name forgot that in the novel that first uses it the Doctor says it's a pseudonym. were friends throughout their time at the Academy, a fact that was hinted in Classic Who and confirmed as part of the canon in The End of Time. The endless Foe Yay - particularly where Ten and Simm!Master are involved - seems to support this.
Dr. Cox and Carla from Scrubs. It was later confirmed that they went on one date.
Some parts of Stargate SG-1 fandom have a working theory that Samantha Carter and Cameron Mitchell were an item at some point in the past.
In Tales of Symphonia slash fandom, if Kratos/Yuan is not happening in the present, it tends to be something acknowledged as having happened in the past.
In more mainstream shipping, one theory about why Sheena and Zelos have the Slap-Slap-Kiss dynamic that they do is that they dated or at least were mutually interested at some point in the past, but one or the other broke it off/put on the brakes before they could get started, and neither is really over it.
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.