This is old news but I wanted to address this because I am a Lost fan and Lost is ending soon. In the first season finale, "Exodus Part 1" we see a flashback of Michael and Walt on the morning of the flight. Walt turns on the TV and is watching Power Rangers SPD. The episode took place in September 22, 2004. The specific footage was of the episode "Beginnings, Part II", premiered on February 2, 2005. This mistake was caught by Power Ranger and Lost fans alike. The producers were always about no mistakes, I wanted to catch at least one mistake, I was glad with this mistake, since it was Power Ranger related. Lost producers clearly did not noticed this, they had access of the footage through Disney, but probably didn't realize the show was time-sensitive. Oh it is not the only mistake--apparently on the episode, a cameraman (part of the film crew) can be seen on the beach filming the survivors attempt to launch the raft.
UPDATED JULY 28, 2010
"Ji-Yeon" Season 4 Episode 7
I didn't notice this. This picture is from Lost: Nerd Winks - UGO.Com but they mistakenly put a picture of the sentai Turborangers which never became Power Rangers. In this episode, Jin goes and looks for a Panda stuffed toy and in the toy aisle behind him are Power Rangers Mystic Force and Operation Overdrive toys.