The kind of reliability I'm talking about where "100% reliable" is a reasonable assumption is inside the boundaries of a single server, i.e. a CPU, memory bus, and I/O peripherals including a NIC and non-volatile storage. Yes, things can and do fail inside that boundary, but if you start taking that into account things get much more complicated. You are also no longer in the realm of distributed systems at that point, which is the context in which the controversy about exactly-once delivery arises.