I found a workaround. This appears to be a bug (“feature”?) in jQuery 1.10.1. Using jQuery 1.10.0, the error no longer occurs:
<iframe src="http://endorkins.com/test-iframe-1.10.0.html"></iframe>
Strange. Very strange. If anyone knows the reason why this is happening in 1.10.1, and how to fix it, I (and jQuery minions around the globe) would certainly be very interested to know! 🙂
UPDATE: Looks like this is a legit jQuery 1.10.1 bug: http://bugs.jquery.com/ticket/13980
UPDATE: According to @emanuele-greco, this is fixed in 1.10.2 and up. So, upgrading your version of jQuery will likely fix the problem.