The RSS feed for a site I maintain recently stopped working. I did a little investigating and found that the
wp_head hook contents of several plugins is getting injected at the top of the page, causing an XML parse error (obviously).
I'm really not sure what the problem is. I'm not really familiar with how WordPress generates its feeds, so I don't have much insight there. I recently upgraded the blog to 3.8, but the upgrade was done yesterday, and the guy who brought the issue to my attention made it sound like it had been going on for longer. I don't know for sure though, it certainly could be a 3.8 bug.
I've tried different themes, tried disabling plugins. When every plugin is disabled, the feed works, because nothing is hooking onto the
wp_head hook that's causing the problem.