I get really annoyed and feel bad for the person who was invalidated because I know how that feels.. These days, though, I'm better able to calm down as I have been validated so much over the years by my therapist and some other people I've had the courage to open up to.. I might wait till the moment's passed and then tell the person who was invalidated that I believe them. I'm really scared of confrontation, but I hope to one day have to courage to just calmly tell the person who is invalidating someone that they don't know what they're talking about and it's not their business to try and dictate what's gone on in another person's life. They can only listen, or walk away if they don't want to do that.
I'm also learning it can truly be hard for some people to see another person's point of view. There are also such horrible things that sometimes happen to people that it can be extremely difficult for others to hear about it. Like, my friends would tell me the things that happened to me are not possible, which was extremely hurtful when I still needed that validation, and at the time I got really angry, but I've since understood they didn't mean any harm, they just couldn't handle something so bad having happened to someone they care about. It never feels good when someone doesn't 'see' you, but it's not necessarily them being mean and wanting to hurt you, they just cannot see what you're trying to tell them.. Realizing other people are 'only human' too has helped me to not get as upset as I used to. I'm working to get to a place where I can just calmly tell them, 'That's not how it was' with a smile

No need to get too upset, just keep correcting them, and if they simply cannot accept what I'm telling them, I think it'll make my life easier if I can accept their limitations. But to get to this point, I think you need to work through any doubts you have in your own mind about how things were, and to receive enough validation from elsewhere.