End-of-year retrospectives usually list what you've learned. I want to do the inverse. Here are the things I used to say with confidence, and have quietly stopped saying. No drama. Just: I no longer believe these.
'We will fix it later'
Later is a place that doesn't exist on most product roadmaps. The thing you wrote 'TODO: revisit' on in 2019 is still there in 2026, except now it has a customer relying on the bug.
'Just one more abstraction'
The abstraction you add to handle the third case is almost always wrong by the time the fifth case shows up. I now wait until the fifth case and refactor into the actual shape, not the predicted shape.
'They will read the README'
They will not read the README. They will skim the first heading, run the install command, hit an error, and search the error message in your repo's issues. Optimize for that user, not the imaginary one who reads documentation in order.
'A senior engineer would never do that'
Yes they would. They have. I have. It's how we became senior, by doing the thing, watching it bite, and remembering. Sneering at juniors for the mistakes we made at the same career age is the cheapest form of seniority and it should embarrass us when we catch ourselves doing it.
'The product team does not understand'
The product team understands fine. They are weighing different things than you are. If you don't know what those things are, that's the gap to close, not theirs to close.
Twenty years in, my biggest skill increase has been in the things I've stopped saying out loud.