An interesting bug in Infocom's Sorcerer

Yes, after all these years, I’m still collecting Infocom bugs. My list is still here, but it’s way overdue for an update. I just wanted to share a bug I found last night.

Not all releases of Infocom games were bug-fixes. I’ve long been curious about just which ones were.
Consider these three consecutive versions of Sorcerer: 6/840508, 13/851021, and 15/851108. It seems a lot of time and work went into version 13, and indeed numerous major differences from 6 to 13 show up using txd or reform. But that 18-day difference from 13 to 15 is pretty suggestive, and the changes seem minor and superficial. But there is a nasty bug found only in version 13.

Actually, the bug doesn’t show up until the endgame, after Sorcerer’s infamous time travel puzzle.
Here’s the background, a bit spoily if you haven’t finished Sorcerer:

When you go down the lower chute to Lagoon Shore, this is your last rest stop before what is arguably the most unfair part of an unfair old school game. Progressing further requires you to enter the lagoon, which ruins your spell book without any advance warning. You can leave the intact book on the shore, but there’s no reason to expect to need to. Otherwise, there is no way to memorize any spells after this point. Finishing the game with a ruined book requires you to have known just which spells you needed to have memorized ahead of time.

This is all intentional game design. But the bug in version 13 makes things even worse.
Once your spell book has been ruined, any attempt to cast a spell generates the message, “The spell no longer readable.” This happens even with the spells you “know by heart”. So not only is it impossible to memorize any spells; it’s impossible to cast any. The bug was obviously hastily patched in version 15; the ungrammatical message is still in the code (even in version 18), just apparently no longer reachable.

Finding this bug arouses my curiosity. I try to imagine what was happening at Infocom.
Even though changes had been made to the code for ruined spells, those changes must not have been tested before release.
I wonder how many copies of the buggy version shipped, and what kind of panic ensued when the bug was discovered.

Does this make the game unwinnable?

It is still possible to win version 13, but you would have to entirely avoid ruining your spell book. Depending on the order in which you do things, you have to cast at least three or four more spells after the point at which you’re likely to ruin it.