Nightwing and Starfire guest-star in Flash #81 (by Mark Waid, Mike Wieringo and Jose Marzan Jr)...
it was a simple but fun team-up story (that exists in a strange place in DC continuity, which I will address in a future column) but it also served as the set up for Waid that Flash and Nightwing were buddies, which led into Flash Plus Nightwing #1 (by Waid, Brian Augustyn and Eduardo Barreto), where we see that they are actually BEST FRIENDS! They go on yearly vacations together, just the two of them! It's a whole other relationship burst out of thin air!
"Diggety dank?" Oh, Dick...
And they were close friends from that point forward, until Wally was erased from history during Flashpoint. And it really just came out of nowhere like that.
It's a minor retcon, but it was a noticeable one.
Thanks to Bill for the suggestion!
Okay, folks, feel free to suggest more examples of this sort of thing! Obscure ones, famous ones, whatever! Send your suggestions to email@example.com!