Ok cool. So that's an implementation error, but indeed not in the implementation of the lightning protocol, as the POS doesn't really do anything with lightning, except present you a QR and wait for a central server to signal that its paid. The error isn't user error though, it's in the payment software between server and terminal (if I remember your description from the other day correctly, a race condition between payment received and cancellation acknowledged.)
Ok cool. So that's an implementation error, but indeed not in the implementation of the lightning protocol, as the POS doesn't really do anything with lightning, except present you a QR and wait for a central server to signal that its paid. The error isn't user error though, it's in the payment software between server and terminal (if I remember your description from the other day correctly, a race condition between payment received and cancellation acknowledged.)