Weizhong approached me in #site17 to point out that user unknowngriefers vandalized the sidebar to SandboxII, adding the following message to the page:
TO ALL VETERAN/HIGH RANKING SCP-MEMBERS :
UR SCPs ARE NOT INTERESTING AND EVEN IF IT IS, IT STILL SUCKS
DON'T CRITIQUE SOMEONE BECAUSE THEY MADE A "BAD TONE"
THEIR EFFORTS OF WRITING SCPs USING THEIR WRITING SKILLS ARE TO BE PROUD OF.
EVEN IF THEY MAKE SOME MISTAKE, DON'T JUDGE THEM. THEY WANT TO LEARN.
THOSE RULES ARE FUTILE. AND IT ALSO SAID "DON'T BE A DICK".
JUDGING BY THE CHAT AND THE FORUMS, YOU'RE ALL DOING THE SAME THING.
They were apparently trying to hide this message in the code by using [!-- and --], but forgot the closing --].
I reverted to the previous (unvandalized) version. I'll leave it to the Disciplinary Team and/or the Admins to decide what needs to be done.
Edit: Doesn't look like much can be done. If this isn't a sockpuppet, they're not a member of the main wiki and the sandbox wikis are open to anyone. If it is a sockpuppet account, I don't know if there are ways to find out who's behind it.