I think I need more help understanding this. In terms of ACID, a consistency issue is something like, for example: "The client ordered an item, and the count of item availability was reduced by 1, but no actual order was saved in the database. An item is now 'lost' with no trace of where it went and how it disappeared."
There is even another consistency hazard right in this very example: "The count of item availability was supposed to be reduced by 2 because 2 items were ordered by different clients at a same time, but a race condition (the two transactions both individually reading the same old value before writing back the new value) led to the item count only reduced by 1. There is now an extra item 'available' that does not exist."
The canonical example is bank accounts and money transfers, but I figured I'd use something that was more likely in the domain of "99% of organizations" (though that also includes things like ERP, where a lot of other canonical examples apply).
Care to elaborate how transactions are "inherently useless" in a web service here, and how the consistency issue happens "between client and server" communication?
Presumably, in this example, the client/server communication triggering the transaction is the order of an item. There are not that many things that can realistically go wrong in that communication (with checksums on different layers, and nowadays even cryptography, guarding against most arbitrary corruption): The order was not received, or the order was received multiple times. In both those cases, the database remains internally consistent: Yes, the order happening 0 times or 2 times may have been wrong, but the database overall shows the state of what you would expect after 0 or 2 orders. No item has been going inexplicably "missing" or "created" until the next inventory, and the external issue can just be corrected by reissuing or canceling the orders, again with the prospect of an internally consistent database. And even a corrupted order would not change that.