Split Into Grid adds printable lines instead of guides
In Split Into Grid window there's an option to "add guides". But instead of guides lines are added that have standard stroke — printable lines that are conveniently in neutral greenish color 0,1 pt stroke and look like guides so it's easy to miss that.

-
Lola commented
Huh, so I guess it's our life now.... Two people missed it and only by a wild stroke of luck another person noticed it before ruining hundreds of euros worth of material...
Thank you for the answer and the advice, I'll check the script for sure!
-
Not new, yeah. This happens in older versions as well. I checked CS6, and it does exactly the same. Quite confusing, never liked it. Thanks for the report!
As a workaround, I suggest using this script that is more honest and is more powerful (paid, but if you need it often, worth it):
https://www.youtube.com/watch?v=yI1WLRb4tkw