The only reason to worry is if you believe that your accessing of this document is being tracked by the authorities ...
Is this a commonly-discussed problem in the security world? Is there an exception granted for publicly-disseminated information (such as leaks published in newspapers)?
"Data spills" sometimes result in punishment. If the mods read this comment, please return the classification to the title as it would ensure people with clearances were warned about the classification of the information contained.
A fairly pointless precaution, as the prohibited material appears all over once it's publicly disseminated, including in places like the NY Times. I worked in a government lab when the first round of Wikileaks happened, and I got the memo: don't search out the material, don't read it even if you find it in the newspaper, etc. But the security people weren't completely crazy, so if you visited the front page of the Washington Post and there happened to be some stuff on there that was still classified, it wasn't treated as a security incident.