I accidentally saved a plugin with update.esm loaded. Will this break my mod for some people, and is there any way to only make it use skyrim.esm as master file?

Im not too sure what your saying, but whenever i mod, i always make sure TO have update.esm checked off. This (i believe i read) just makes the CK more compatible with the update.

I was wondering about this too. Unless there is a good reason otherwise, I think it makes more sense to have your mod dependant on both Skyrim.esm AND Update.esm. It seems the downside of dependancy on both ESMs is that it might break the mod for a small handful of people that haven't updated their Skyrim, but the upside is that your mod will be based on the current version of the game. The latter seems far more important.

Hmh, I read that update.esm will always be loaded by Skyrim anyway even if it's unchecked in the Data Files section. Thanks for the answer, I think I'll keep it to Skyrim.esm for people who don't want to update yet.