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.)

optimism

In this case I’m saying the POS machine as the user of the network made an error in accounting. One might also say it was me and the clerk for not watching more closely what the other was doing. Lightning just did its thing. The accounting implementation by the POS was just too cutting edge and still had a bug.