I said "nous" as /nou/ for two months until I saw the IPA: /nu/. What French word were you mispronouncing without realizing?
Confession: for my first two months learning French, I was saying nous as /nou/. My English brain saw n-o-u-s and rounded it into something that sounded reasonable to me but was probably painful to anyone listening.
Then I saw the IPA: /nu/. There's no "o" in there. The audio had been telling me /nu/ all along, I just couldn't unhear what the spelling suggested.
It cracked something open. Take "nous avons des amis heureux" looks like four clean words. The IPA reveals it's three blocks with two Z-sounds that don't appear in the spelling: [nu.z‿avɔ̃ de.z‿ami øʁø].
In your own language learning, has one insight ever "unlocked" a whole category of mistakes for you?