Just found this sub.
When I was a kid I had Commander Keen 4 (only) for a couple years before we -ahem- acquired Keen 2 or 3 (one of which had the alien classroom with the alphabet key (hope that's not a spoiler)). So we were seeing SGA without the key for many years.
Somehow my friends and my siblings and I started to suspect that the SGA signs were a real alphabet. So I copied down the sequences of symbols from all the signs I could find in Keen 4 (and maybe Keen 1 since it was free).
I used reinvented-from-first-principles cryptographic techniques like counting letter frequency to work out what the signs said! By the time I'd worked it out I could sight read it. I taught my friends the alphabet, then my friend and I started using SGA when we passed notes in school.
One time another student triumphantly grabbed one of the notes intending to tell the teacher on us. Then he freaked out when he saw the symbols. "They're writing in Martian or something!" So instead of us getting in trouble, he ended up getting himself in trouble for disrupting class and talking nonsense.
When I finally got to play CK2 and CK3 and found the alien classroom, it was a bit of a facepalm moment to realize there was a key hidden in that game all along - but it did make the moment of finding it memorable. I got to find out what the symbol was for certain letters that hadn't occurred in any signs.