My ckeditor installation suddendly stop working correctly. Any time I clicked on a button that should open a pop-up window (link, image...), the whole page turned to the standard white opacity, but no popup window showed up. You could neither escape with Esc, you got stuck and need to reload the page and loose your changes.
I was using Pressflow 6.28 + WYSYWIG + CKeditor 3.x (not the latest). Trying to chase down the bug, I tested first with the last ckeditor 3.6, then uninstalling wysywig and going with this standalone ckeditor module, with 3.6 and then 4.0.1. Same result.
Uninstalling modules and stuff, I finally verified that a block with the official twitter feed widget was the one to blame. (tested in 3 domains with 3 different blocks)
For the sake of documenting and avoiding some fellow headaches ;)