Ash, why didn't you tell me about this thread?!
This happens to me all.the.time. They get so stuck in my head that I start singing them. Which can be bad. Case in point: "I'm not their fool, I'm not their n*****" from Ragtime.
Another Ragtime one- "She had life in her, lord she had my baby."
Rent "Nothing, your smile reminded me of-" (sang as Manley Pope- the way he says "nothing" has this bizarre accent to it)
It rotates between lots of songs, but I always keeping coming back to the same few lines.
Like a firework unexploded
Wanting life but never
knowing how