Every few years Donald Knuth raises his head from his travails on TAOCP and considers the state of TeX. From time to time people submit bugs they’ve found in TeX and during these periodic respites from his life’s work, Knuth addresses those bugs. He considers TeX as converging to a perfect state and uses these periodic reviews as an opportunity to advance that convergence.
The latest cycle happened in 2014 and resulted in a few non-user-visible changes to TeX. The real lesson here is how mature TeX is. There are no glaring, document-destroying bugs. Rather there are a few cobwebs in some obscure corners that scarcely anyone has ever bothered to visit. Consider Knuth’s TeX Tuneup of 2014 for the latest changes. As you can see, there’s nothing in there that would give you the slightest pause.
It’s received wisdom in our field that software is never defect free but TeX is a candidate to refute that. It may not be perfect but it’s certainly converging on it. Tex is surely one of the best pieces of software ever written.