DOS was my favourite. Most people around me would have "win" in their autoexec.bat file so that Windows (3.1 I think) would load automatically at start up. Not me: Word Perfect for DOS was all I needed. I only loaded Windows when I absolutely had to. I was a power DOS user, and knew all the commands, and wrote complicated batch files.
Two years later, I realised that everyone around me seemed to be a Windows expert, and I could scarcely use the mouse. I had a lot of catching up to do. By then, we'd all been forced to switch to Word for Windows where I worked. People around me were writing macros and drawing diagrams in Word, but I was struggling, trying to type a capital N but accidentally pressing Ctrl N rather than Shift N. This produces a blank document, and there was me wondering where my report had gone. Undo (by then I'd found out about the undo command) didn't undo the blank sheet of paper.