The woman that interviews Locke at the agency (before Rose), is the same woman that pretends to be a psychic in Tricia Tanaka Is Dead (2007), the woman that Hurley's father paid to convince him not to go to Australia.
Each of the names written in the cave has a number next to it. These are the doomed numbers seen through the entire series. 4-LOCKE; 8-REYES; 15-FORD; 16-JARRAH; 23-SHEPHARD; 42-KWON.
The sound of John's alarm clock is the same alarm that went off in the Swan hatch when the time was getting close to 0.
Hugo claims to now be the 'luckiest man on earth' when speaking to Sawyer on the plane during LA X: Part 1 (2010). When John Locke gets fired from the box company owned by Hugo Reyes, he is unable to access the wheelchair ramp of his van. In anger, he tries to lower the ramp and dent/scratch the huge Hummer parked next to him, but the ramp stops an inch short of inflicting damage, much to the chagrin of a furious Locke. The Hummer belongs to the 'lucky' Hugo Reyes.
The man in black (played by the same actor as John Locke) says "Don't tell me what I can't do"- one of John Locke's signature phrases.