Semi-mandatory geekery: GPS was available in the late ’90s, but not much at the individual civilian level. So Jack’s curse/post-hypnotic suggestion/paradox filter isn’t just preventing him from mentioning things that are unknown in this timeframe; it’s preventing him from hinting at things that would raise questions.

But it’s not perfect. If Shelly was a geek, “so you know how to get to places … from the sky” might cause her to say “You mean GPS?” – which would presumably cause that element of the block to crumble. The longer Jack keeps trying, the more likely he is to find ways round this thing.