On loading any content page as a logged-in user, there is a brief flash of an empty plugin. (Possibly more noticeable if you have click-to-play on in adblock or browser.) This is part of swfobject.js's browser capability detection.
Any reason the inclusion of swfobject.js is not conditional on the "Use Flash upload" setting?
Any reason not to use a version of swfobject.js that fixes this issue, issue 357, reported about five years ago?