Like when you get into a conversation with someone about the dog's name on the show Empty Nest and they say it was Belvedere and you say no, it was Dreyfuss and you argue for ten minutes and then one of you looks it up on Wikipedia and they realize you were right and then you say:

 "I told you so".

 Or like when you tell your husband that you can't watch Transformers 2 because you'll have nightmares and he says you'll be fine, not to worry and you watch it with him anyway and then you have nightmares for two weeks about rogue machinery trying to hatchet you into pieces which you then relate to him in great detail and say:

 "I told you so." 

 But there are times that you don't want to say those four little words (though they might slip out on accident because you don't think before you speak and you apologize in advance for any times that has happened in the past or may happen in the future).

 Like when you tell your kids not to eat that fifth cupcake because they're going to get sick and they ignore you and eat it anyway and then you wake up in the middle of the night to them vomiting up chocolate and icing mixed with stomach fluids.

 You don't really want to say "I told you so" at that juncture, though you rightfully could.

Because you did.

 Or when you give someone relationship advice and they don't take it because they think they need a woman/man right this instant even though you know they aren't ready and you tell them so and then the whole relationship thing doesn't work out in the end and they tell you about it and say that they, in fact, weren't ready.

 You might want to say I told you so, but even if it was said in the nicest way it will still come out sounding jerk-ish and not nice and totally not encouraging but you may not be able to think of one other thing to say that might be encouraging or helpful so you say it anyway and add a :) to the end.