The game sayd it was having 'Error adding key Bookshelf1 to dictionary: an item with the same key has already been added.
I had not been trying to add a key to my dictionary, or doing anything else. All I had changed, after pressing stop game (I am proof reading a "book" on a "bookshelf"), was to revamp some of the message that acts as the text on a certain page.
I don't know anything about adding keys and dictionaries except that the book is on Bookshelf1, but I hadn't touched it, even in the game. I was already turning pages when it popped up.
I hope this is probably fixable.

What command had you used when the error occurred? Some built in commands use dictionaries internally for some things.
Did the error message quote the line of code that caused the error?
Were you typing a command, or using the menu?

I'm trying to work out where this issue could be coming from, but I can't figure it out. Most of the dictionaries used by the core libraries are either created in the editor, or on game startup, so shouldn't be having items added to them during the game.

I was in the margin, pressing the verb "turn page" I looked and looked into that verb, which still seemed fine. I couldn't find anything relating to the bookshelf. Maybe I had, and do not remember it, pressed "bookshelf" and "look at", since that was its only verb, but that would have led to text, not a script.
I went ahead and deleted the bookshelf, and made a new one with a different name, just in case there was actually something attached to the name somewhere.