The rule could be anything, as funny or as serious as you want. The universe will progress in a similar way that it has up until this point, unless your changed rule prevented it from doing so.
Some examples might be:
- The invention of currency is not allowed.
- Iron is slightly less stable.
- The Ancient Greeks are able to cultivate Silphium, which does not go extinct now.
Cats speak french, except in any situation where they can be recorded or transcribed, or when a french speaker can hear them. They also aren’t very good at speaking french, but it’s impossible for anyone to know that.
What if two English speakers are listening to a cat speak French, when suddenly they are joined by a third companion who speaks native French?
The cat stops talking.