Gnucash is a double-entry accounting system.  The user will expect to see at least two accounts be affected by every transaction.  Gnucash has the very nice ability to hide accounts from the tree (automatically for 0-valued accounts and explicitly).  While this is really a nice feature that can aid in getting a quick overview, I see a potential for confusion when one end of the transaction is on a hidden account and the other is not.

I propose to introduce a by-default-on warning for the following actions

a) entering a transaction on a hidden account
b) hiding an account which has transactions on the book