Back tire was low, not flat, when I got to my bike yesterday afternoon. It had been locked up (in my bike locker) since Friday morning, so I figured I had a slow leak; pumped it up and rode away. Two miles later it was flat. Pumped up and rode away. Didn't get a hundred yards before it was flat. So I got out my tool kit and found, oops, no tool kit. Someone owns too many bikes and at some point moved the tool kit to another bike.
I had to call my wife to come get me. That was a first; fifteen years of bike commuting, she has never come picked me up before.
Anyway, I took out the tube. It had six patches on it, three of them all in a group. One patch was actually partly over another, and the next one was just a few milimeters away. I couldn't find any external cause for the puncture, so I suppose we should regard this as a failed patch, but
my last flat tire on this bike was in December. The patch took a good long time to fail, but its failure was predictable. Lesson learned? Carry a tool kit.
There was no way to patch it again, so I put in a new tube. And I stuck some tools in the handlebar bag.