Someone prompted me to make a warning sign for causality violation. I came up with these two. And a related "closed space" warning sign. Made very quickly, hence the wacky font; I re-used the basic warning sign template from a team fortress 2 spray I'd made.
I think the one with the question mark is the best. It looks properly geometric as well as symbolic. or at least it would if it was in a serious font. I could easily see it become the kind of sign you end up recognising as a holistic image rather than a combination of symbols. I like the idea of the closed space one, but am unimpressed by the picture.