When your wiping your ass and you finger breaks through the toliet paper.
It should be noted that there are diffrent catagories of blow through.
category 1. You get some shit on your finger.
category 2. Your whole finger goes in.
category 3. Your hand goes in up to the knuckle.
category4. The most extreme case of blow through in which your whole hand enters the rectum.
My roomate got the cheap TP and I had a case of category 2 blow through.