In that case, it's precisely what you're thinking.
So I've been experiencing breakthrough bleeding, for which I inserted a menstrual cup -- a small Lady Cup -- before bed on Sunday. When I woke up Monday morning -- because it was Monday and because it was morning -- I thought I'd spent the night cup-free and inserted my "all day" cup (a large Lady Cup).
Yes, it was difficult to insert. No, it was not comfortable, really. The large cup definitely sat lower than ideal -- but that's not totally unusual for me and anything larger than my small Lady (at least that I've tried). So I had a vague notion of something being off but no idea that something was totally wrong until about noon on Tuesday.
I'd been changing the outer cup (what I'd thought was the only cup) every 6-8 hours. When I emptied my cup during the day today, I noticed: 1) it was basically empty; 2) it stank way too much for being in for only 6 hours. So I inserted a finger into my vagina, wondering if the smell was coming from my vag rather than from the cup (thinking that this might mean infection). Lo and behold, an extra cup!
Yeah, the one that's been in ~36 hours smells like my dog's breath after she's been eating cat shit. Gonna vinegar and sunlight that.
I'm not terribly worried about my vagina (several hours after cup cup removal, everything is back to normal) or my cups (I can unstink them just fine). I'm just sort of curious -- has this happened to anyone else?