Let's be honest: there are a million places a piece of music can be folded* to mono. A couple quickly come to mind: you'd be amazed how many video editors don't understand the difference between stereo and mono. If it has two channels, many think that's enough. They'll import a music clip and the software breaks it from a single stereo track into two mono tracks. They'll often leave the two tracks panned to the center and export it.
Instant mono. The same thing happens daily in broadcast facilities everywhere. You call 'em up:
Me: "Hey, this clip is mono!"
Editor: "But it's on both channels!"
Me: "But that's not stereo!"
Editor: "Huh?"
Bob
* Folded, spindled, mutilated, etc.