Question on headphone jack

[armus2112]

I put on my Sony headphones plugged into my second-generation iPod Shuffle this evening. "I Can't Tell You Why" from the studio album started playing. But it was ONLY the rhythm guitar. There was -- no singing, no bass. Just the guitar. Like I was hearing the raw guitar track from the studio. After 20 seconds, I could barely make out the vocals "Look at us baby..". In fact I didn't even know it was "I Can't Tell You Why" until the very faint vocals started. I then jiggled the headphone plug and made sure it was inserted all the way and I could ear the song as normal then.

I was perplexed. How could I only be hearing the guitars and all the rest of the instruments were nearly completely absent (very faint).

Later in the day, I listened to the song on my computer, thinking maybe it was the left track or the right track and the headphone jack wasn't plugged in all the way. But no - the left/right tracks contain more than just the guitars.

What was going on there, anyone know? I thought there were only 2 tracks on a stereo mp3.