Honestly, did you have zero contact with Karen between her asking Sarah to pass on the invite and the supper? Or with anyone else who was attending the supper party?
I know if I was Karen and in casual conversation with the OP I'd at least ask "Oh, btw did Sarah mention the supper party on Friday only I hadn't heard if you could come?" and then if the OP had no clue, I'd be thinking "Sarah must have forgotten, good job I mentioned it!" and invited the OP and if the OP knew then I'd be thinking "Oh good, Sarah remembered, now I know OP is definitely coming/can't make it"
I also know in my friendship circle that, if we're all going to someone's house for a get together, at least one or two of us will send a message asking if anyone needs/wants a lift or to walk over together or to check what time we're all geting there or to ask what drinks everyone is taking (we usually all bring a bottle to share, but try to co-ordinate so we bring a selection- and we don't all rock up with a bottle of vodka and no wine) or any number of other questions ... yet the OP hasn't spoken to anyone who has even mentioned it in passing?
And of course, on the night when everyone else was there and OP wasn't did nobody ask where she was if she was expected? Did Sarah fess up that she'd forgotten? Even if that was the case did nobody call and say "So so sorry! Miscommunication meant everyone thought everyone else had told you about tonight but turns out nobody actually did ask you! Anyway, we're having a girls night - just supper and a few drinks, any chance you can make it for a couple of hours? If not we'll definitely have to get together for coffee!"
Sounds like you're geting Wendied in this case.