I recently had this problem IRL: tried to pay in a coffee shop with LN, but my channel partner was "sleeping", so after a while I paid in cash. Then, 5 min later, the server comes back online, the LN payment went through and I had to go get my cash back.
lightning-wallet.com doesn't seem to have a stop button, but I think this (IMHO important) usage scenario is just not possible in the protocol right now, is it? The BOLTs say: "For simplicity, a node can only remove HTLCs added by the other node. There are four reasons for removing an HTLC: the payment preimage is supplied, it has timed out, it has failed to route, or it is malformed.".
So I guess the question is, why is this not possible today? Just a UI problem, a protocol shortcoming that will be fixed, or is there a deeper reason why it CAN'T be possible?