The book states this saying twice, word for word: here and at 16:25. It is also one of the most quoted verses in Proverbs.
Quick answer
Being sure you are right is not evidence that you are. The verse names the one error a person cannot detect from inside it.
Read both lines
The whole force sits in "seems right." This is not describing someone doing what they know is wrong; it is describing someone doing what they are convinced is correct, which is why no amount of sincerity is protection. The second line uses the same word for "end" that appears in the very next verse about laughter turning to grief, so the two sit together as a pair about outcomes that were not visible at the start. That is also why the book keeps recommending counsel: a road you cannot assess from the inside has to be assessed by someone standing outside it.
Why translations differ
Nothing here is disputed, but the repetition is worth knowing about. Proverbs states this saying identically at 14:12 and 16:25, and the Hebrew is the same in both places. Repetition in this book is not editorial carelessness; the collections were assembled from real sayings, and a proverb that mattered enough to circulate twice generally earned it. Nothing about the meaning changes between the two locations.
What it does not mean
This is not a verse for dismissing other people's convictions while exempting your own, which is the standard misuse. It is stated in the third person about "a man," and every reader is that man. Nor does it teach that a person cannot know anything or trust any judgment; Proverbs spends thirty-one chapters urging people to develop exactly that. What it establishes is narrower and harder: confidence is not self-verifying, so a wise person builds in the checks that certainty by itself will never supply.
Related: Proverbs 16:25 · Proverbs 14:15 · Proverbs 12:15 · Isaiah 55:8–9
New King James Version®, Copyright © 1982, Thomas Nelson. All rights reserved. Full copyright information. Scripture provided by API.Bible.
Go Deeper
If this verse has been used to put weight on you that it was never carrying, ask about it directly.