I've found an odd error in Safari (I'm on 5.0.3 but have noticed it on late 4.x versions)
See the attached screenshot - intermittently in admin in Safari JS code will show up behind the UI.
I haven't noticed the same issue in Firefox or Chrome, but in Safari it seems to be triggered by switching from the asset manager to the site tree view, but as it's intermittent it is difficult to pin down a definite cause.
This is on SS 2.4.3 but I've also noticed it on older version such as 2.4.0. It doesn't seem to be module related as it's occurred on sites with no modules installed as well as sites with bags of installed modules.
Any ideas? I'd love to be able to ignore it but a few clients do use Safari to manage site content and it looks dodgy when the bug pops up in the middle of a CMS demo.
Happy to put some time into hunting down the cause if it is a genuine bug.