Unwanted video elements show up in media export

For some reason, a completely unwanted animation element has found its way onto my (near) final export of a video. I have reviewed the timeline and the thing in question is not anywhere at the timecode. In fact, it's an extremely uncomplicated bit of video besides. This weird bit just 'found' its way there and since I can't find a corresponding erroneous clip or anything, I don't have any idea how to test this and stop the behavior.

I'm happy to provide additional data, but right now don't know what I could provide...