Professor Stephen Hawking once famously organised a party for time travellers at the University of Cambridge, but only issued invitations — including date, location, and time — after the event. In the real world, the fact that no one turned up was perhaps further indication that time travel is impossible. If Hawking’s past self within the Star Trek universe was left waiting, however, it was probably because time travel exists but is by and large a terrible idea!
One problem with time travel, not exclusive to Star Trek, is that no matter the manner by which you mean to achieve it, things will likely end (and begin) in paradox. If you don't wind up facing that one famous patricidal self-contradiction, then it will be 'Pogo,' 'Dalí' (or the 'melting clock'), and never-ending causality loops. Forget trying to get your tenses straight — "the past is the future; the future is the past" — and prepare for one headache after another!
After and before all that, if it’s not the effect that gets you, it will be the cause. Out of all the ways to go about a temporal displacement in Star Trek, very few, if any, are sure-fire or safe. In fact, the vast majority are fraught with danger, are downright disastrous, or just damned dodgy, and there are plenty that didn't make this list.
No wonder there was a war fought over it!
10. Romulans And Radiation Exposure
If you're interested in using the 'O'Brien must suffer' time travel method, you're going to need two things: a dose of delta-series radioisotopes from an exploding plasma conduit and a quantum singularity in orbit of your location. You'll meet your future self and then some! Must remember: Avoid panels on the habitat ring, and basilar arterial scan! Basilar arterial scan!!
The mode of time travel in Visionary might well be distinctly unadvisable, but it does work as a clever enough central conceit for a fun episode. It allows O'Brien to hop into the future with just enough, but not too much, regularity to keep the mystery going — and, ultimately (decades-old *spoiler alert*), to save the Station from destruction at the hands of those scheme-loving Romulans in their warbird. The real problem is the damn deadly radioisotopes. The 'original' O'Brien of the 'present' succumbs to radiation poisoning and is replaced with his future self, thus joining the duplicate club alongside Harry Kim.
The freelance writer Ethan H. Calk, who came up with the idea for Visionary, had a thing, it seems, for terrible time travel methods as he also has a story writing co-credit on the episode Children of Time. We all remember how dreadfully disastrous the temporal tamperings were for Gaia!