Linux

RedHat 3.0, kernel 1.2, early 1996. I was a contract developer and took a job for a customer to update an in-house curses app on SCO Unix. Aside from a few lab uses in college, I had never used Unix before. I was like, welp, I'll just install RedHat, do the work there, and recompile the app at the customer's site on their SCO machine. Stupidly charged into a massive learning curve (unix vs linux, gmake vs make, gcc vs cc, ncurses vs curses, ... none of which I had any familiarity with), but, amazingly, I got the job done! Kept RedHat as a second boot option on my workstation, and continued to use it more and more... 30 years later, I'm typing this on a MacBook Air running NixOS.