Dateline: Blocky Mess
In recent months a problem has emerged for some YouTube users. Usually it presents itself in the form of unexpected blocking artifacts in the video as YouTube renders it. The source of the problem appears to be related to changes YouTube made to increase efficiency of uploading and processing, by doing the work in parallel. I'm not sure of the details, but in essence this suggests to me that checking file integrity must be using some test that is not nearly as reliable as more traditional checksum methods. I can see your eyes glazing over.
Suffice it to say, that the way to test whether your uploads are affected is to upload a file that is known to convert reliably, at least from users whose connections are not affected by the current glitch.
If you want a file or files to test, I've created two of them. One in 1080p, the other, much smaller file, in 360p resolution. Downloadable from Vimeo:
1080p version Compare the YouTube version here.
360p version For comparison, here's the YouTube version.
Both videos are provided for use under Creative Commons license
The works linked above are licensed under a Creative Commons Attribution-NoDerivs 3.0 Unported License.
If you confirm this problem is affecting you, you have at least two current options.
One: try to change your connection to one as direct and uncomplicated as possible. For instance, if you are uploading from a WiFi connection, try uploading from a machine that's wired directly to the appropriate router/cable modem (whatever is most direct in your situation). Since this may not be possible for shared connections or in many other situations, and the specifics will tend to vary, YouTube recently coded an uploader meant to resolve this connection-related problem, which gives you Option...
Two: Try using the SSL uploader developed specifically as a workaround for this known problem, which hopefully will get a more elegant solution in the not too distant future. The link is https://upload.youtube.com/my_videos_upload (note the SSL "https:" prefix is the key difference, and may be the only difference you'll notice between the SSL uploader and the standard uploader).
Please be aware that some users have reported it taking much more time than their previous uploads. You may want to take any steps possible to ensure that the connection is not being used for any other purposes during the upload, to reduce chances of failure and avoid what slowdowns can be avoided.
This issue is discussed extensively in the latter pages of this YouTube help forum thread (the same link as the dateline at the top).