There was a webcomic (or something like it) I saw a while ago that was about a single-buttoned gadget that could always predict whether you will press the butto

There was a webcomic (or something like it) I saw a while ago that was about a single-buttoned gadget that could always predict whether you will press the button or not (don't remember the exact mechanism, but that was the jist). The story then went on about how the gadget would bring about humanity's collective philosophical collapse, because of its implications on the subject of free will. I imagine that what you're describing about superluminal communication could be used to make a gadget like this. I'm also wondering if anybody knows which webcomic I'm referring to. It'd be embarrassing if this turned out to be an XKCD number...