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.
Ice fishing is now just called fishing.
It also becomes much easier to make perfectly clear ice at home: you pour the unfrozen, cloudy water right off the top of any partially frozen thing.