"For He's a Jolly Good Fellow" is used at the Chief's birthday party because the "Happy Birthday" song was too expensive. In September 2015, "Happy Birthday" entered public domain and it can now be sung anywhere for free.
In this episode a malevolent entity possesses Keiko's body, holding her hostage to force O'Brien to help her. While speaking with O'Brien, the entity in Keiko jokingly suggests that the crew will likely try to catch her in some sort of stasis field. In the episode Power Play (1992), Chief O'Brien's body was possessed by a malevolent entity who similarly held Keiko and Molly, then still a baby, hostage. He was briefly forced from O'Brien's body when caught in a sort of electromagnetic stasis field.
It was during this episode that Nana Visitor went into labor with Django, her son by Alexander Siddig. An explanation was written into the story that Kira was visiting her lover Shakaar on Bajor.
First time the Pah-wraiths are mentioned.
This was Allan Kroeker's first Star Trek directing job. Kroeker would go on to direct almost forty episodes across DS9, Star Trek: Voyager and Star Trek: Enterprise including the finales of the fifth, sixth and seventh seasons of DS9, the finales of the sixth and seventh seasons of VOY, and the finales of all four seasons of ENT.