The 1984 Doctor Who story Resurrection of the Daleks was originally broadcast as 45 minutes episodes, the first story to do so. However, when it was repeated, each episode had been edited into two 25-minute episodes.
I've just watched all of the Fifth Doctor stories, and I noticed that Resurrection of the Daleks (the 25 minutes per episode version) has slightly different title music from all the other Fifth Doctor stories. The most noticeable difference is that when the Doctor Who logo appears, there is a slight audio cue, similar to but much more subtle than the Sixth Doctors original title music.
Where did this version of the title music come from?