I don't see a 404, but two images are loaded twice: one with the timestamp and the second one without it.
http://nightly.ckeditor.com/5829/_samples/replacebyclass.html
http://nightly.ckeditor.com/5829/ckeditor.js
http://nightly.ckeditor.com/5829/_samples/sample.js
http://nightly.ckeditor.com/5829/config.js?t=A7M74HW
http://nightly.ckeditor.com/5829/skins/kama/icons.png?t=A7M74HW
http://nightly.ckeditor.com/5829/skins/kama/images/sprites_ie6.png?t=A7M74HW
http://nightly.ckeditor.com/5829/skins/kama/images/dialog_sides.gif?t=A7M74HW
http://nightly.ckeditor.com/5829/skins/kama/editor.css?t=A7M74HW
http://nightly.ckeditor.com/5829/lang/es.js?t=A7M74HW
http://nightly.ckeditor.com/5829/skins/kama/icons.png
http://nightly.ckeditor.com/5829/skins/kama/images/sprites_ie6.png
http://nightly.ckeditor.com/5829/contents.css
http://nightly.ckeditor.com/5829/plugins/styles/styles/default.js?t=A7M74HW&t=A7M74HW
So we should certainly look at the reason of this problem.
I guess that the ones without the timestamp are loaded due to the stylesheet references.
And maybe the patch to avoid zoom problems in IE can expand this issue to all IE versions.