My Nexus S does the weirdest thing when I'm walking home and listening to podcasts with Google Listen. Occasionally, it just jumps to another podcast. Today, it cycled quickly between three different podcasts. Usually I pull it out of my pocket, remove the headphone plug from the jack and blow into it.
This would make sense if I had a headset with more than one button, though PowerAMP takes control of the single button on my headphones (Meelectronics SP51P), so the worst that should happen is my music starts playing instead of my podcast.
A bit of pocket lint in my headphone jack shouldn't be causing these weird problems. The only other factors that I could think of is static caused by the cord rubbing between my sweater and jacket or having one side of the phone warmed by my leg and the other cooled by the outside air.
Google Listen might be the problem, so I could try an alternative if there's a half-decent free app available.