Never the Selves Shall Meet: Averted. While it’s difficult and expensive – to understate the case radically – to meet yourself (albeit less difficult, although no less expensive, to send yourself a message) in the past, and it comes with a host of annoying limitations concerning boundary conditions and suchlike, there are essentially no consequences for doing so. The universe is block – various chronology protection theorems point out that the probability of any event-chain that will create a global causality violation is zero – but it only cares about global causality violations, which is to say, as long as there are no uncaused effects or effect-free causes out there, it doesn’t care about the order they come in. You can create predestination paradoxes (with certain difficult-to-manage limitations; for example, looped objects cannot age, otherwise the loop loses end-to-end consistency and becomes impossible) all you want. It does, which is rather more annoying to the naïve user, mean that you can’t change the past – whatever present you are in incorporates the consequences of whatever changes you made in the past even before you reach the future in which you travel to the past, so nothing can be changed since you already changed it, if you did.
More sophisticated users, which mostly means those weakly godlike superintelligences again, make great use of the ability to whisper instructions in their present self’s ear from the future via acausal logic processing, but it’s still imperfect and of limited bandwidth, so it’s not actually the quick ticket to omniscience it might seem like.