When I was about 4 years old, I have this vague memory of watching my cousin playing Super Mario Bros. and coming across a level that was an underground castle with multiple Bowsers. My adult self accepts that the memory is so fuzzy it may have been a dream, but I spent many, many years looking for this level and was utterly convinced that it was hidden somewhere in the game. After all, there were so many secrets I knew were in the game, like warp zones, that surely there could easily be more. Now that rom editors exist, I could actually probably recreate that level, though I don't really remember it now, aside from having multiple Bowsers.
When I was like 10 or 11, I knew enough about computers to understand that programs were a series of instructions, but I didn't know exactly what programming was at the time. Therefore, I was convinced that by altering random bits of data in a .EXE file, I could rewrite a computer program to do different things. I always made backups, so I never ruined any games I had, but damn was that a waste of time. Then, I discovered programming from a book I got at the local library, but the book was written in the 80s, and so assumed that you could just type BASIC into the command prompt. Took me forever to figure out why typing in BASIC commands to the DOS prompt didn't work.
Finally, when I was 12, I discovered Doom, and more importantly, I discovered that it could be modified. My first mod ideas were incredibly laughable - I had no idea how the game worked or how data was stored, and so I was convinced that I could do simple changes to turn stairs into ramps and turn zombie guns into flowers. Once I learned how editing worked, however, there was one other flaw - I was convinced that I could somehow trick the shareware version of Doom into running add-on levels. Ultimately, that never went anywhere, as I quickly found that I couldn't bypass the shareware level lock, but it did lead me to designing a number of levels which I intended to play once I got the registered version but which turned out to be utterly unplayable because I had no way of testing them.