I know I told the story at VP, and it was years ago, but I'm not sure if anyone has ever had this experience before.
Proviso -- I didn't do this on purpose, I don't recommend it AND I am 110% fine afterwards.
This was back when I was using both a large and small lunette interchangeably, roughly 5-6 years ago. At the end of my cycle, I put in the small lunette, stem cut off (my cups tend to ride a bit sideways). Then I forgot about it.
I had finished my period more or less, and just didn't remember to take the cup out. I always had trouble with it coming out as they'd tend to ride /very/ high, especially the small one.
During that week (it was probably about 10 days as near as I can figure) I had sex a few times, with a penis owner wearing a condom. He didn't notice, I didn't notice, I still don't know how that happened (they weren't a long term partner, so that possibly contributed to them being less thorough).
The only way I noticed was I started getting a strong smelling, chunky discharge a bit like yeast, but definitely different. I went to the bathroom at the train station on the way to work and checked inside myself (as I usually do when I notice something strange). I felt the bottom of the cup. 'Oh NO.' I thought. 'OH NO THIS ISN'T...HOW LONG...OH NO.' I didn't panic but I definitely facepalmed.
I braced myself and removed the cup, which was now sitting low enough for me to grab. Warning for graphic language from here, but I figure you would have stopped reading if you weren't curious at this point.
The /worst/ smell of my life came out of the cup. It looked like I'd actually put it in after all the blood and tissue had passed, so it was just the regular discharge that had accumulated for that nearly two-week period. It stung a little as it flowed out of me, and as I said before, absolutely /stank/. I felt a bit traumatised, but relieved, and more than a little grossed out at myself.
I wore a disposable pad that day and drank loads of water. I carefully cleaned myself out with clean fingers that night in the shower and .... was fine. I had bad smell for about 2 days before the flora got itself back to normal.
I want to tell this story because if I had left a tampon in there, I would possibly be dead from a systemic infection, have required emergency room care or have had other serious complications from leaving something to rot inside me. I was incredibly lucky that my absent-mindedness let me off the hook so easily.
I disinfected and washed the cup but it never lost a tiny bit of that smell until I sunbaked it in the Australian summer for a good week. I still never used it again, as there's no way I could forget about the much larger sized cup.
I've experienced some degree of that smell twice since, both with unrelated instances of bacterial vaginosis. They were completely cleared up by antibiotics. I think I was pretty lucky to not need further care in that previous instance.
And that is the story of why a few of my close friends, when I am looking for something, ask me 'Oh, have you checked your vagina?' >____> Never living it down.