Question for StarCraft 1 experts. Experts only, please.

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An opponent's worker (SCV) entered my main base
and then left (my gas extractor was already built when it entered).
Naturally, I should have had no vision after that,
but then, while my Dragoons were blocking the entrance,
it just passed through as if it were invisible.
Of course, the Terran had no vision.
It happened exactly as if I had clicked on a mineral patch while having vision.
Looking at the replay, it wasn't a different SCV,
but that same worker that had left came back in.
What I'm curious about is:
1. Is this possible without using hacks?
2. If it is possible, is it a type of cheat? Or is it a legitimate technique?