I understand. As it’s “intended to help users avoid mistakes”, a better error message might be helpful…
It wasn’t immediately obvious to me that the rules were incomplete - given that all cases were in fact covered. Obviously, I didn’t need the “Cask” condition, but it seemed useful in a self-documenting kind of way. Perhaps some kind of non-conditional label node (?) would be useful in cases like this. Or the ability to add a label to the outcome / leaf / best before. e.g. “Then it must be cask” in my example. (have created a FR for this: Best before rules - add label to outcome - Ideas & feature requests - Breww Community)
Incidentally, the date preview does seem to work even for a tree with an “open end”. Which is a bit bug-adjacent…