Earlier quoted context omitted.
> nonzero If you send the message with a hash of the previous state on the server, (like with proof-of-work in Bitcoin), since it is so unlikely that it will hash will be the same with and without the message appended, it doesn't really matter if it is strictly nonzero, if it is just small enough.
The nonzero part is about the possibility of network / endpoint failure. No hash is going to prevent the cleaning lady from 'freeing up' the power socket for her vacuum.
You could resubmit the transaction until you get an ack. You wouldn't submit twice since the hash of the state with the added item is very likely to differ.