I practically learned OOP making an application in Turbo Pascal using the Turbo Vision library. I even bought the Pascal source code for Turbo Vision ($200 IIRC

I practically learned OOP making an application in Turbo Pascal using the Turbo Vision library. I even bought the Pascal source code for Turbo Vision ($200 IIRC) Primarily to find a bug where when you pressed a pull down menu item, and there was a button on the window below the menu item, the mouse event for the menu item also triggered the button below. It turned out there was a patch downloadable from a Borland BBS, but boy it was learning digging through the source and finding and fixing that one inlined assembler opcode which was wrong. Dissecting the patch one could see that it contained a workaround, not a direct fix of the bug. Today there's a lot of open source to learn from. Not at all common at that time.