You don't trust me :) then just try the below steps:
1) Copy any text by Ctrl+c or Edit >> Copy or anyother method which you are comfortable with!
2) After that just visit this Link: http://www.friendlycanadian.com/applications/clipboard.htm
3) You will see the text you copied on the screen right there in that page.
Hope you would realise the seriousness now. The thumb rule is do not keep sensitive data (like passwords, creditcard numbers, etc.,) in the clipboard while surfing the web. It is extremely easy to extract the text stored in the clipboard to steal your sensitive information.
Its always advisable to either copy useless piece of information onto the clipboard before leaving a shared computer OR logoff / restart the machine so that clipboard content would be erased.