The original game simply played cards at random from its hand, and was pretty easy to beat.  Then I realized that there was a simple strategy -- if the player asked the program for, e.g., a 5, the program remembered that the player had a 5.   If it later drew a 5 it immediately asked for it.   This "pro" version was very hard to beat, to the extent that nobody wanted to play it.  So I made the pro version an option--the default was the dumb mode.

It didn't get a lot of hype, but I did face an irate user once at a Usenix meeting who publicly accused me of cheating (since the program did, in fact, know what the player's had was).  The pro option was that good, but, unless somebody changed a copy of it, the user's hand wasn't part of the strategy...

Looking at the code a couple of months ago, I found at least one bug and one logical error.  The bug would have been caught by Lint, but that program was many years in the future.

Steve