If an arithmetic combinator has just one channel connected you can glitch the interface deselecting active channel: the other channel checkbox unlocks.
It happens both for input and output channels; this does NOT happen for decider combinator.
Is it possible to remove the locking behavior entirely? It's annoying that you always have to connect duds before you are able to change settings, remove the dud wire, connect to active wire. I'm in favor of having an indicator that nothing is connected there, but I prefer having no indicator at all than having to go through this for every change I make to an active system.