I told you so.
That expression should die. It should be thrown out of the lexicon, in every relationship.
I find it hard to think of another phrase that manages to fold two of the lowest human phenomena into two words: the urge to come out in the right, and the need to demonstrate your superiority over somebody else.
But leave the wretched intention aside. What is genuinely broken about the expression is the result.
There is no more effective way to drive people away from you and to make them develop contempt and resentment towards you.
And it is worth being precise about why, because it is more interesting than mere rudeness.
The person you are saying it to has just had something go wrong. They are, at that exact moment, engaged in the only useful activity available — working out what they got wrong and what they will do differently. That process is fragile, it requires a degree of honesty with oneself, and it is only possible in the absence of an audience keeping score.
I told you so converts their private reckoning into a public defeat. And a person defending against a defeat cannot simultaneously be learning from it — so the sentence not only wounds them, it reliably prevents the very outcome you claim to have wanted.
You were right. That is genuinely nice for you.
Say nothing, and you get to keep both the relationship and the possibility that they take your advice next time. Say it, and you have traded both for three seconds of being acknowledged.