State of the art in L2 transactions means that you can send a payment without needing to issue a L1 transaction. There's a small, defensible risk that the sending party could try to broadcast outdated state in the L2 transaction (trying to roll back the state) but the protocol is designed with counter measures to deter that. Basically if the receiver isn't online or all the miners DoS the counter measure (retribution transaction) then the receiver is at risk. It's very small but still ought to be known.
