Luke Champine

You go back in time and tell [person] that [bad thing] will happen if they don't [do something]. As soon as they are convinced, you vanish, because in the new timeline, there was no need for you to go back in time.

Unfortunately, you vanish *precisely* at the moment that the person is convinced -- mid-sentence, actually! This abrupt disappearance shocks and confuses the person, instilling a degree of doubt... you see where this is going.

Each time the person swings doubtful, you reappear to persuade them again; but you can never remain corporeal long enough to convince them permanently.

Right up until the moment [bad thing] actually occurs, your poor subject is still wobbling between 49% and 51% doubtful. Despite your repeated interventions, you've been unable to reduce the chance of catastrophe lower than a coin toss.

So I show them this thread and they’ll know I disappear the moment they’re convinced!

That *might* work. However, we should probably assume that any objects you brought back in time will vanish along with you. So you start showing them a print-out of this thread...but you and the print-out vanish before the person can finish reading.

Perhaps you could win by writing on contemporary paper with a contemporary pencil. You'd just have to be careful not to do so in the presence of the person; otherwise, you would likely disappear halfway through writing.

Hm wouldn’t my writing disappear too?