Some people get songs stuck in their heads, I get words or phrases looping over and over. Sometimes I'll get both a song and a word repeating in separate loops at the same time! I think it's a similar mechanism though, it's a lack of closure. I believe they repeat because my brain is looking for connections, or trying to learn something from it. The annoying ones are when I can't remember the ending of a song, so the middle keeps looping over and over. Or if I can't quite remember the definition of a word, very annoying. The best ones are the ones I can remember all of the tune but it surprises me every time.

Edited for clarity.

To me the annoying ones are the ones I can't seem to get rid of, no matter how hard I try. I've got the a short vocal fragment from an Annie Lennox song stuck in my head for days at the moment so I feel very connected to what you just wrote.

Aluminum!

Aluminium!

Good grief, I can't even spell it!

If you're not being sarcastic: http://www.worldwidewords.org/articles/aluminium.htm

Now that was fun, thanks. I find it annoying that I am taught to spell a certain way, but then the British keep reinventing spellings with "s" instead of "z" and adding an "i" in there just Americans on their toes.