When someone asks me about an event that I was not invited to, the last thing I'd do is say that I wasn't invited. I usually just say something like, "It sounds like fun, but I already had another commitment that day." It is a little white lie and probably more evasive than need be, but the one time I responded honestly, it led to months of awkwardness at work. Years ago, I made the mistake of saying that I wasn't invited to a coworker's event when "Bess" asked me if our coworker, "Angie," had mentioned to me which weekend they had decided on. Bess made a minor stink to me about how rude it was that I hadn't been invited. It was awkward, but no biggie. I wasn't offended since I wasn't close with Angie, I had other plans that weekend, and the event wasn't my cup of tea.
A few days later, Bess saw fit to tell me that when she called Angie out on not inviting me, Angie flat out said, "If Sneezy comes, I'm not even going to go." That was a biggie. First, why put Angie on the spot like that? Angie should have invited whoever she wanted without interrogation. Second, I had previously existed in blissful ignorance of Angie's disliking me. This was seriously a non-issue until Bess made it one. I think she meant well, but...ugh.
The fallout was miserable. Previous to this incident, we had a decent working relationship. We had nothing in common, so we'd never become friends, but she was okay to work with. After this, Angie probably thought I was the one who was offended, rather than Bess being offended on my behalf. Or maybe it was just too awkward to deal with me after being called out like that by Bess. Or maybe she felt that since the cat was out of the bag, there was no reason to continue to fake being civil to me. Or perhaps when Bess mentioned the event in front of both of us and I said, "I hope the weather's good for you this weekend," Angie thought I was being passive aggressive about not being invited. Maybe it was something unrelated, like sending her an email in the wrong font. I have no idea what the cause was, but Angie was extremely unpleasant to work with after that event. While it wasn't quite like being in Hades, it certainly fit neatly into one of the outer circles described in Dante's Divine Comedy. Asking if I did something to offend her and offering to help clear the air got a dismissive sniff and a denial that there was a problem at all. On her last day, she pointedly crinkled her nose at me on her way out the door.
I learned my lesson. If a third party mentions an event of any kind, I beandip the question of being invited with a reference to another commitment that day.