When I was 10 or so, I "cracked" Slam! Air Hockey for Windows 3.1 by opening the exe in EDIT.COM and replacing some random binary garbage with spaces. After a few attempts, I managed to bypass the shareware dialog but also introduced some weird bugs that I don't recall the details of.

"Cheat enginge"

This was one of those things you really really wanted but once you toyed with it, it sucked the fun out of games and they felt pointless.