Maybe 1 in 10 failure rate or less, it's not that common. Harder to patch successfully on the inside of the tube. This is with 26x2.125, which gives you more surface to work with, and that may help.

Something else is that some tubes are quite a bit smaller than the tire they go in. In that case, I try to inflate the tube to about the finished size before sticking the patch on. It kind of dimples when the tube is deflated, but then doesn't have to stretch when installed. After sticking the patch on, knead it good with your fingers. Good luck.