I like to call this the, "Practical Jokes" proverb. That is because it describes what can happen when practical joking gets out of hand. Unfortunately, I've watched a few of these in my day and they can get ugly in a hurry.
The proverb speaks to us of a certain madman. This guy is out of control. He is throwing three things in his insanity. The first is firebrands - which are akin to something like a flaming arrow. This crazy guy is also shooting regular arrows. The third thing he is dealing in is death. The first two are easy to understand, but this last one is a little more cryptic. I see the final thing in light of what happens due to the deception. He is throwing around these things that hurt physically - but he is also throwing something that hurts emotionally, relationally, and spiritually. He is throwing "death" - he is killing relationships and injuring people so that they are dead to him emotionally. His actions are hurting things and killing someone's ability to be around him - interact with him - and receive anything from him. So we see three pretty rough things going on here. All three are destructive and can cause great harm. The interesting thing is that since he is a madman - his aim may not be the best. From how this is stated, it seems as if a rather random pattern is being followed in how these things are being shot and thrown. Thus he will not hit everyone, but when he does, it is going to hurt badly. It might even kill someone. What could this be describing? Let's look, because the answer is given in verse 19.
Verse 19 tells us that the comparison is to a man who deceives his neighbor - then tells him he was only joking. First of all we need to see that this man does these things to his neighbor. The term here implies more than just a casual relationship. Some passages imply a relationship as close as a close friend or even a lover. What he is doing to his neighbor is that he is "deceiving" him. The word here is "ramah" and it means to intentionally deal craftily with someone. Other ways it is used is to indicate lies, betrayal, crass jokes, and even pulling a trick on someone. That is definitely the case here in Proverbs 26:18-19. What is going on is that a trick or a crass joke is being played on a person by his neighbor, which is pretty much the definition of a practical joke.
If we did not know the nature of this proverb through the word "ramah" - things become much clearer as we see that after this man deceives his neighbor - he eventually lets him know about it by saying, "Was I not joking?" There it is - a practical joke, pure and simple. But why is this such a strong statement? Is God adamantly opposed to all practical jokes? From what I read here I cannot say one way or the other. This is just a warning about consequences.
A day is coming when the practical joker is going to play a joke on someone and it is going to blow up in his face. He is like that madman randomly shooting arrows. Most will fall relatively harmless to the ground. They won't hit any real target. But every once in a while he will strike something - actually someone. When that happens - problems are coming. In some cases real harm comes to someone in a practical joke. Somebody gets physically hurt - and at times it is a bad injury. In other situations the pain is much worse - because it is emotional and relational. I've actually watched relationships broken forever or for a long time because of a practical joke gone bad. These are very sad things to watch because the joke was meant to be funny (at least to the one doing it - and to everyone watching the person humiliated). But at times the humiliation breeds anger. That is why God warns us about it.
In all honesty - when I consider that most practical jokes are done to humiliate someone - or at least to laugh at their expense - I think that a wise man should probably avoid them. He knows that such an action may result in an offended brother. And from other proverbs he knows that if this happens that brother will be very hard to win back. That is why the wise man decides against such actions. But before you think I am a 'stick in the mud' on this issue - you should know that I have been the mark of many practical jokes. How do I respond them? I laugh with those who did them . . . sometimes threaten retaliation . . . and usually end up forgetting them and thinking, "You got me good!" It is good to have a sense of humor - and an ability to laugh at yourself. Just remember that some won't - and when that happens - it's going to be bad - very bad.