Here is a perfect example of the principle mentioned: I wrote an answer to this some hours ago but, just as I was about to click Submit, my laptop blacked out. There went my answer. Now, since communication is unrepeatable, I can only hope to give a vaguely recalled rendition of my first (really good!) answer or hope to re-think the question and come up with something as good, though it will of necessity be different. Since communication is inevitable, I desire to try again rather than raise my hands (which is communication itself, though seen by no one but myself) in despair saying, "I'll never get that back!" and going on with the next DB thread. Since communication is irreversible, well, I guess this one doen't apply to my laptop blacking out my answer ....
One thing that is meant by communication being inevitable is that we are communicating beings. We each have thoughts stirring in our minds that we desire to communicate out loud to an attentive and understanding listener. We desire to be understood by someone and to understand someone. Whether the communication be overtly verbal or through the flip-side of the coin, signing as in American Sign Language. And, yes, communication is also unintentional as through microexpressions, subtle gestures, attitudes, and non-specific responses.
Communication is irreversible and unrepeatable for the same physiological reasons. Communication sent and received creates new chemical and electrical pathways through the synaptic neurotransmitters (noradrenaline, for one) in the brain. These pathways are irreversible, though science has recently shown they can be muted. These pathways are also unrepeatable because, as suggested above, if you try to repeat a communication (either a sent or a received communication) you are actually repeating the memory of the communication and memories, though sometimes near-perfect, are always still a memory and not the original communication. If you re-think the communication, it by definition cannot be a repetition.
It is for these physiological reasons that communications, even seemingly simple ones, do have irreversible effects on the cognition, memory, brain structure, and life events of both the sender and receiver. This is why the more practical explanations that lead to wisdom like "Think before you speak," "Appear wise through your silence," and "A hasty word cannot be recalled" come into being. It is for these same reasons that we owe it to ourselves to be attentive to communications because some of the most important ones are small, simple, and subtle and ... unrepeatable. If we fail in this attentiveness, we will find crucial, though unrepeatable, communications lost forever, sometimes with life altering effects.