The BBC's Sherlock is great. But it's been bugging me since I first watched it. Now I know why.
Sherlock main theme:
Final Fantasy IV Main Theme:
I can't be the first person to have noticed this. I hope this helps solve other folks' earworm problems.
Update 9 January 2014: I replaced the original embed of the FF4 theme with a new one as YouTube had since pulled the original.