it could be that someone is using youtube on a different computer that you have used before and you used the rember me option on both computers in that case the other person will be having the same problem as you but might not realise it!

that or your password was hacked and someone is logging in to your youtube account without your permission

there is also this help articule on youtube that could help if it not one of the above
http://www.google.com/support/youtube/bin/answer.py?hl=en-GB&answer=116919

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