Wordpress | Blank First Line in RSS feed | Broken RSS feed

We've just had this happen to TWO sites: The first line of the RSS feed comes out blank, meaning that the RSS feed is broken. Feedburner can't cope, so the RSS subscriptions, Twitter feed, and e-mail subscriptions all disappeared down a black hole. This happened after a couple of major site revisions, involving new plugins. First up, we tried the "fix-rss-feed" plugin. Nope. Then the "whitespace" fix. Nope. Removing blank lines from wp-config. Nope. Hmm.. The solution is the obvious, boring one: disable ALL plugins, and re-enable one-by-one until the RSS feed breaks. Turns out it was nothing to do with the new plugins, it was "Advanced Tag List", which I think has just been updated.. It just goes to show - you can never be too careful. With muktiple plugins by multiple authors, this IS going to happen. Anyway - everything is again rosy in the garden.