I saw an article about the copyright dispute between singer/songwriter Joe Satriani and the band Coldplay. Satriani, according to the reports, thinks that Coldplay's song "Viva la vida" copies from Satriani's song "If I could fly."
I decided to test my skill at creating a musical air on the 4-chord underlay that Coldplay use. These chords, transposed into the key of G, are C, D, G, E-minor. (Analysis on YouTube here). So I laid out four repetitions of these chord changes and wrote a melody, and, wouldn't you know it? Parts of my melody sound superficially like parts of Satriani's "If I could fly" and Coldplay's "Viva la vida".
Here is my "Air on 4 chords:"
Before I began writing I had heard parts of both songs a couple of times, but I could not have sung them back. So unconscious copying is not likely unless my memory for popular airs is better than I thought!
Maybe Coldplay copied from Satriani. But I think Satriani might need to consider the possibility that two independently-created airs on similar chord progressions can often have points of fortuitous resemblance to each other.