It was 1991. I was working in the virus lab on a bunch of stuff sent in by various people.
Each candidate file got copied to my "infectable" computer, an old IBM PC clone without a hard drive. Then I ran the COM or EXE file. Then I ran my "goat" files, several tiny programs that only existed to get infected. Once one of more of them were infected, A) I knew that it was indeed a virus, and B) I had the virus isolated in that infected file.
By a year or so later, I automated that process, using Novell Netware 2 to store the candidate files and feed them one at a time, as if from a hopper, onto the infectable computer, run the file, run the goats, filter off any that changed, re-image the infectable computer and on to the next candidate. It made things a lot faster and more efficient.
But in 1991, I hadn't automated things. So I ran the candidate file. And that was the first time I encountered Tetris.
Three hours later, I was still playing the game.