The cast were supposed to sing "Happy Birthday" to Worf in Klingonese, but the producers couldn't afford the asking price by the song's copyright holders. It was just too high. They felt it wasn't worth it so they substituted "For He's A Jolly Good Fellow Instead". Apparently it was a lot cheaper.
Although he doesn't say his age, according to the Star Trek Fact Files, Worf turns 30 in this episode.
When Data's painting changes, its a depiction of a Vor'Cha Klingon Attack Cruiser.
In one reality, we see a Cardassian ensign at the helm of the Enterprise, the only time we see a Cardassian in a Starfleet uniform. With the Bajorans a more militant race here, and having conquered the Cardassian Empire, it suggests their roles in this universe are reversed.
The communicator badges used for the reality where Riker is Captain and Worf is First Officer are the same badges that were used for the future scenes in Future Imperfect (1990).