pool #6596 -- 2013 Type-Moon Calendar (Morii Shizuki)
Unless having exactly one unrelated image in that particular pool is a meme, this repeated vandalism needs to be stopped somehow.
We don't have the ability to lock pools yet, but this might be due to a bug or something? Just throwing out a thought.
This is happening because it's the first (active) pool when listed alphabetically, so it's the default option when you click "Add to pool > See full list". The problem's now moved to pool #6332, next in the list.
I'm guessing pool #10702 (named ".") was intended to catch these misfires, but it's marked inactive so it doesn't work.