I taught myself x86 assembly from a book as a pre-internet teenager. My first .COM printed exactly what it was supposed to on the screen. Then it printed a bunch of random junk and my computer rebooted. This happened every single time I ran the executable.

Eventually I actually read the first chapter properly and added MOV AH,4CH / MOV AL,0 / INT 21H. When my executable returned to the DOS prompt this time after running it, I felt like my mind had just expanded 10-fold as I realised what had been happening.