Note: Originally, this topic was about me having a problem with my video playing. However, seeing the comments on various videos I've been testing on, this is universal.
Currently, YouTube seems to be having major problems for a large amount of users. While videos can play when loaded, trying to move along the timeline will result in a black screen and eventually an error message. This effects many/all videos currently. If anyone finds any official notice, be sure to link it here so I can add it to the OP.
Google's Known Issues page, listing whatever current issues YouTube has, both long term and short term, which are effecting users:
https://support.google.com/youtube/bin/static.py?hl=en&page=known_issues.cs
