When Yan and SP Wong are waiting at the elevator, the digital floor counter skips the 4th floor. In China and Hong Kong, the number 4 is considered bad luck because it sounds similar to the word 'death'.
When referencing this film as the inspiration for the Best Picture-winning The Departed - Il bene e il male (2006), the announcer at the 79th Academy Awards mistakenly identified the Hong Kong production as Japanese.
The psychiatrist's name, "Lee Sum Yee", is a homophone (in Cantonese) for "your psychiatrist".
The film had been selected as the Hong Kong entry for the Best Foreign Language Film at the 76th Academy Awards but it was not nominated.
It was Hong Kong's biggest box office grosser of 2002 ahead of Harry Potter e la camera dei segreti (2002) and Spider-Man (2002).