When Rishi Kapoor meets Kirron Kher, she says her nickname is Bobby. He says, "Why don't I call you Dimple?" The eponymous female lead in Bobby (1973), was played by Dimple Kapadia.
Vivek Oberoi was chosen to play Karan Kapoor, but he declined. Aamir Khan and Hrithik Roshan were also offered the role respectively, but both of them declined too.
Rishi Kapoor plays Saif's father and sings a song while at the Paris airport. The song, "Mai Shyer To Nahin" (I am not a poet) is from his famous movie Bobby (1973), as are many other inside jokes in the movie.
Hum Tum was a hit at the box office and made Saif Ali Khan a bankable name. However, the makers had different plans for the movie. Apparently, the makers of Hum Tum had started off with a very long title, which was Na Jane Kab Pyar Ho Gaya. However, it was later changed to a short and sweet sounding Hum Tum. Also, when the film started off there was apparently a cameo offered to Lara Dutta. However, according to IMDb, the actress was busy shooting for Khakee and hence had to opt out.
Saif Ali Khan plays a very quirky character in the film sporting colourful clothes and funky hairdos. One of Saif's dialogues in the film was, "This is my Tom Cruise look." Apparently, there is a story behind the same. According to IMDb, one of the assistants had said that one of the looks was like that of Tom Cruise. Director Kunal Kohli heard it and decided to make it a part of the script. While Saif didn't seem to be in support of it, Kohli managed to convince him.