One of my friends, while generally fun to be around, is prone to telling a lot of bullshit, specifically bullshit that revolves around her. And while I have my suspicions, it's usually not the kind of thing where I know she's lying, so I let her get it out, give a neutral grunt of acknowledgement, and change the subject.
A few weeks ago, she spun one of her usual tales to me and two other friends, but this time it was about someone we all knew. None of them had ever really spent time him, but we used to be pretty good friends and I know him fairly well. The story made him come off as a major creep, but while this friend is socially awkward to the point where someone might believe it, he would never act the way she said he did. I could smell the bullshit from a mile away. The problem was that while I was 99% sure she was lying, all my evidence was still circumstantial, and he lives far enough away that it didn't seem worth the fight. So I vented to one of the friends we were with via text, she agreed with me (even though she doesn't particularly like the guy), and the conversation moved on.
But now, three weeks later, I realize that there is absolute proof that she was lying. Abso-fucking-lutely proof, that I completely forgot about until now. So she slurred my friend's name when I could have proven his innocence, and I did nothing. And if I try to call her out on it now she'll probably deny it or try to change the details of what she said, even though there were two other witnesses and one of them almost certainly remembers it. I'm mad at her for saying such terrible things, mad that she used his genuine lack of social skills to make it seem true, and mad at myself for letting her get away with it. What a shitty way to treat my friend behind his back. GRRRR.