Undecidability
I've done that before. Once in high school my period started during the middle of the day so I got a tampon from a friend. When I got home I was bleeding a lot so I put another one in without taking the first one out. Before bed, I took the second one out and was like WAIT A MINUTE and found the first.
GOOD STUFF. I made the opposite mistake...normally my period is pretty heavy at the beginning, but there actually is no normal for my period so I missed a month and this month it was like ten seconds of spotting but I was like "geez better put a tampon in just in case" because I was going to be out for a while, and I was in a hurry so I didn't realize I'd already put one in, and the lack of blood didn't tip me off because I wasn't really bleeding.
My uterus is really stupid. But at least it and I have that in common.