This is a weird thing that's happening now that I can't really explain is that I have Radio Mode on. I opened up xpTunes to the Titanium template and went to Playnow. It displayed only the files that were currently in Playing Now, as expected. I clicked the next button in xpTunes and MC started playing a new file. xpTunes didn't display this file in Playnow. If I click out to some other place and come back to Playnow, then it shows. But even as I'm typing this reply, MC is skipping to the next track, playing it for a while, and then skipping to another track. MC is minimized to the system tray, and xpTunes is in another tab in this browser.
If I then restore MC, it seems to stop, but if I minimize it again, and click next in xpTunes, this behavior persists.
Actually, I get the same behavior whether Radio mode is on or not.
Shutting down xpTunes seems to clear this behavior up, so I have to assume it's some weird thing with xpTunes sending the next file command over and over again to MC.