Renee O'Connor did "Déjà Vu All Over Again" and "Dangerous Prey".
Michael Hurst, who starred on Hercules: The Legendary Journeys and made several guest appearances on Xena directed quite a few of the better done episodes. In fact, he was such a recurrent director, that the cast and crew, (all the way up to the executive producers) started to call him "Cup-Of-Blood-Michael" due to his love of using fake blood, and penchant to want more and more. You can thank him for the episode "To Helicon and Back"; one of the most gruesome and violent episodes in both the show's history, and that of television.
Disowned Adaptation: Rob Tapert openly hates the Dynamite Comics arcs, which act as a Fix Fic to developments that Tapert supported and continually defends to this day. One interview also suggests he's annoyed that Dynamite simply procured the licensing rights and didn't request any consultation (as he makes a point of criticizing them for doing that with their Army of Darkness comics).
Dye Hard: Lawless and O'Connor are both natural blondes; Lawless dyed her hair black (on Hercules) then brown, while O'Connor dyed hers red for most of the show's first two seasons.
Flip-Flop of God: Ongoing after the show ended. Mostly regarding the nature of Xena and Gabrielle's relationship. Of note, around the time of the finale, Lucy Lawless felt it was "more than friends" due to a particular scene. By the time of recording DVD commentaries, this opinion was reversed.
Non-Singing Voice: Both Gabrielle and Callisto in "The Bitter Suite," and Gabrielle again in "Lyre, Lyre, Hearts on Fire" (although the latter counts as a subversion - Renee O'Connor is one of three performers who do Gabby's singing here, according to the soundtrack album credits!). Averted with Xena and Joxer in both episodes, Ares in the former, and Draco (and Amoria) in the latter.
Promoted Fangirl: In a move unprecedented in television history, popular Xena fan fiction writer Melissa Good was hired specifically because she was a popular fan fic writer (of the uberficTropical Storm and its sequels, among others). She wrote the sixth season episodes "Coming Home" and "Legacy", as well as a third musical episode that was supposed to involve Sappho as a Gabrielle lookalike, which never went past the outline stage.
Real Life Writes the Plot: The writers really wanted to bring back Velasca, but Melinda Clarke's schedule was consistently too busy.
Running the Asylum: Several episodes of the sixth season of were actually written by a (critically acclaimed) Fan Fic writer who was approached by the creators of the show and asked to write a bunch of episodes.
Viral Marketing: In a very early example, in 1995 the show's website (itself unusual for a show at the time) featured an archive of e-mails between researchers working on "The Xena Scrolls".
Take That!: Charon's entire rant in You Are There where he bashes Hercules for never paying his tolls while comparing Xena more favorably against Herc was more than likely a giant Take That! directed against Kevin Sorbo. By the time this episode had aired, Hercules had already ended and Kevin Sorbo himself had ended his Hercules stint on a very sour note with Rob Tapert. Tapert had wanted to continue Hercules for three more seasons while Sorbo himself was tired of playing Hercules and wanted to move on to Andromeda, in addition to already disliking Tapert, claiming that Rob had taken away money, production value, and his favorite writers away from Hercules to benefit Xena. Sorbo himself admitted that things didn't end well with Rob Tapert in his autobiography and all the things Charon said about Hercules was more than likely a case of Writer on Board where Rob Tapert was venting and expressing his exact thoughts on how ungrateful he considered Kevin Sorbo to be.
A version of the script for "The Furies" would have revealed that Ares was indeed Xena's father (this was changed due to 1) not wanting Xena to be a half-god like Hercules, and 2) the Squick factor that would result from Ares' physical attraction to Xena and even sleeping with her when Callisto was using her body). The final version is ambiguous enough that some people assume this to be the case anyway.
The episode "Intimate Stranger" was suppose to end with Xena getting her body back from Callisto but when Lucy Lawless had injured herself the ending was changed to have her still trapped in Callisto's body so that Callisto's actress could fill in for Xena for the following up.
When Lucy Lawless was thrown from a horse during a stunt on The Tonight Show, it triggered a story arc where Xena was in a death-like trance and her soul had to be carried in the bodies of other characters on the show. Thus, "Xena" could still appear on the show while Lucy Lawless was recuperating. Two of the soul-carriers were Xena's enemy Callisto and Autolycus, the King of Thieves.
Gabrielle spraining her ankle in "One Against an Army" was written because Renee O'Connor really did sprain her ankle during production. It's even acknowledged in the credits.
While not technically an infirmity, the entire fifth season was written to accommodate Lucy Lawless's pregnancy, sidelining her and giving most of the action to Gabrielle and various supporting characters.
You Look Familiar: Many examples, but Mael, Cupid and Julius Caesar, all played by Karl Urban, stand out. Lucy Lawless & Renee O'Connor both played different characters in the Hercules: The Legendary Journeys preseries Made For TV Movies.