I wrote a short little tune to go with a poem from the 16th century. It bugged me and bugged me because, dammit! I had heard first line of that tune before. Finally nailed the sucker and realized it was Deep Purple's "Smoke on the Water". No problem then, changed a note, and it sounds different on the recorder anyway...

Any time I write a song (very, very rarely) I am terrified that I plagiarized it from somewhere. I drive all of my friends (both of them) nuts asking "Do you recognize this tune?" I'm still scared that everything I've written was on a recording my parent's had when I was 3.