Jesus condemned calling people “fools,” but the writer of Proverbs has no such inhibitions. We should not see it as a contradiction. Jesus was condemning an attitude that is contemptuous of others. Solomon, in chapter 26, is speaking of a reality: some people are simply fools. You can tell who they are by the way they live their lives, and you should be careful around them.
They are unreliable (vs. 6).
They are hopeless. They never learn from their mistakes (vs. 11).
They believe they have the answer to the world’s problems (vs. 12).
They are dangerous. I particularly like verse 8. “Like tying a stone in a sling is the giving of honor to a fool.” You can fire a stone from a sling as David did against Goliath, but if you attach the stone to the sling, it will only come back to wound you.
I’ve thought about that a lot when asked, as a minister, to give recommendations for others. To recommend for any reason other than they are commendable is sure to come back to haunt you.