Add a warning, if a booking is in a placeholder account.
Placeholder account typically have no bookings. If they do, they should generate a warning.
I'm not sure yet, were the warning should appear and when, but open for discussion. I think I would not be sufficient to only do this when a booking actually happens (is this even possible? Never tried). For me this usually happens, when I create subaccounts and have forgotten to reallocate all bookings properly.
I guess, the warning could be made optional.
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Aaron Laws commented
It is not possible to post to placeholder accounts (at least in the latest Gnucash, and I assume it has been this way for a long time). To mark an account as placeholder, right click the account in the account tree, click "edit account", check the "Placeholder" checkbox.