What you are saying is mostly impossible. Progressing too much i mean.
Mostly impossible isn't a thing. If it's "mostly" impossible, then it's still possible.
It was entirely possible to unintentionally lock out any of the optional Billy content, even when explicitly following the walkthrough on a fresh game. The options for getting Billy a blowjob at the pool, and consequently everything else Billy related, would not happen. No giving him panties (option wouldn't show up despite having them), literally none of the options for doing anything regarding Billy after Janice's Birthday event.
No cheating, either C+H or Regedit for autosave, just pure, unadulterated grinding with copious amounts of save-scumming. That was on 0.25 Final or 0.26 Final, can't remember which.
If you are using older save, sure but we all know using older saves also cause bugs.
Can cause bugs. Not guaranteed to.
With each new version i always run a new game to see if there are bugs. Never ever bugs occured on my case(of course this is for final version). So each time KsT posts a new version, he also has to do extra programming/scripting to adjust new stuff for your older save games. Which leads to; either longer production for each version or less content for each version.
Final version 0.22, every time I went to the school at 21:00, Black Screen O' Death!
Nothing changed hardware or drivers wise, started a new game to test, same again.
Didn't work until 0.23.
0.23 Final, Janice's new swimsuit caused issues with her tan lines,
if the tan lines were changed from default state, causing the game to stop in that legendary Black Screen O' DEATH! proceeding whenever she was at the pool in the new suit.
Editing save file and restoring tan lines to normal was the only solution. Again, same on a new playthrough instead of just continuing an old save.
Point being, what Ikaruga was saying in regards to how the game handles certain things,
can lead to issues where variables get messed up and affect the game.