Here's a bizarre one for you guys. I was using Search RegEx to remove Blogger html formatting that was screwing up my imported posts. Images hadn't been imported, so they were loading from Blogger (and within anchors pointing to that same Blogger image). Then all of a sudden I noticed three of the six post images stopped loading. I looked in the html, and lo and behold, the images had automagically been uploaded to my media library! I checked my FTP and there they were, thumbnail sizes and all! Somehow they had been added a few minutes prior. Any idea how this happened? I do have the "Add Linked Images To Gallery" plugin enabled, but I hadn't touched it yet.
Most likely "Search RegEx" triggered a filter/action that "Add Linked Images..." was attached to, which caused the import.