To reproduce, login to an account on YouTube and load a new video put up by one of your subscriptions. The gray subscriptions window that slides up from the bottom doesn't seem to have any function. YouTube displays it's loading symbol. I am using Adobe Flash Player 10. Not that I like that feature of YouTube at all, but I thought I'd put this up here, see if anyone else has experienced it.
Is this a new problem? What browser/nightly are you seeing it with?
I'm not sure if it's new, I searched through the bugs before posting and didn't see anything like this. At the time of the original posting, I experienced it in WebKit r65371, but I just installed r65398 and the problem persists. I now found out as well that the drop-down button on a video page next to the video uploader's username doesn't work. The button that shows the uploader's other videos, I mean.
Mass closing plug-in bugs, as plug-in support has been removed from WebKit. Please comment and/or reopen if this still affects WebKit in some way.