Starflight, sure as heck. Part of it might have been poor game design, but it was actually a shining beacon of playability in the mid-to-late-80s of its release.
So you're cruising around interstellar space. You only see the small area around you--let's face it, it was designed for early 286es. There's a starfield scrolling past when you move. If your navigator is high-enough skilled... you notice that there's a patch of stars that, instead of moving with the rest of the starfield, seems to be moving randomly.
If you touch that patch of random movement--or if you touch a patch of totally normal looking space, if you have a poorly trained navigator--suddenly, you are treated to a 3d zooming-through-starfield effect. And your coordinates like "81x136" are suddenly replaced by "???x???". Dude...Where ARE we? You've just hit a continuum flux (IE a wormhole). They're two-way, so it's possible to stagger around and go back into the one you just came out of. But here's the tricky part. Many of them lead into clusters of OTHER fluxes, which means that if you trip over one that you didn't know about...your chances of getting back home successfully are, well, very very low. Tripping over one of these is scary as hell, especially considering that the game is sort of roguelike-ish when it comes to saving (scumming is encouraged, but it required you to manually back-up TWO FLOPPIES).
Also scary about Starflight when I was a kid...just encounters in general. Especially if you're somewhat new to the game and travelling in space you're not already very familiar with, and especially if you've just fallen through a continuum flux, encounters are quite likely to kill you. Anyway, you usually get some text warning that other ships are nearby. If/when they catch up to you, your PC speaker makes three low tones for the encounter bell. And when I was eight years old, that nearly gave me a heart attack EVERY TIME.