I have posted a video on YouTube and it begins with many gray pixels (squares).
What can I do?
See this print-screen too:
migrated from superuser.com Aug 3 '12 at 8:39
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YouTube officially supports WMV files, and the Flash Player seems to work for most videos, so the video corruption is likely due to a bug during the conversion from your original WMV video to the MP4 that YouTube stores on its servers (it may even be as little as one corrupt frame).
You should try uploading the original again to see if conversion will work the second time.
If it does not, you can try converting the video to MP4 yourself. SUPER is a popular and easy-to-use program for this. The dimensions of the video should not matter, so you don’t have to worry about re-sizing or anything. Just convert it to MP4 yourself (which gives you greater control over the quality) and YouTube may not even convert the it at all.
They also have some tips for uploading videos that may help. If all else fails, you can use the Report a bug link at the bottom of the page or contact them about it (the contact page says AV sync issues, but the page that links to it does not: If you continue to experience Audio/Video issues with your upload, contact us and we may be able to help.)