For those just tuning in, in about the past month or so, various problems have shown up for many people, most dramatically, the tendency of HQ videos -- those that have been encoded to YouTube's Format 35 (&fmt=35) -- to play back at roughly half their intended, uploaded frame rate.
So far there's been no real word on what's going on with the High Quality format. I do have it on reliable authority that this issue has been brought to the attention of YouTube's engineering department, but so far no word has gotten back to me that the source of the problem has been found.
The following videos will demonstrate this behavior, at least as long as the problem persists. I tried in this and other videos to duplicate the method used by user kekkomatic and described in detail in his video description.
Basically, an alternating pattern, frame-by-frame of graphic boxes were laid into the video as two overlaying video tracks. The boxes alternate left-to-right, frame for frame.
If there were no dropped frames one would see these as a sort of constantly "shimmering", rhythmically consistent set of images. When frames are dropped, instead one is likely to see the boxes "stick" on one side or the other.
To confirm that this is limited to HQ35 encoding, you can go to any affected video and force display at another quality level by pasting the following to the end of the video URL.
NQ5 - add "&fmt=5" (without the quotes)
NQ34 - &fmt=34
HQ18 - &fmt=18
HD22 - &fmt=22 (where available)
Related videos should include several other demos. Here's a playlist of several tests, including ones that did not get HQ35 encoding and are therefore largely unaffected by frame droppage. (Playlist appears below the fold).
Playlist of test videos, as noted above, appears below. (The preceding hotlink will also take you to the playlist on YouTube itself).
Videos that were not strongly affected by dropped/skipped frames are in the playlist, towards the end. The following is a further attempt to explore issues related to this problem, but it seems to have failed to achieve the HQ35 encoding. It's included here instead to test whether one can truly disable related videos in a auto-generated custom player. Only one video should be playable in this player, if that's true.