Long story short: I put a patch on a punctured road bike inner tube, it was fine for two days and 20 km, then the tire became completely flat, due to air leaking from the edge of the patch; why?
On Saturday, I patched a punctured inner tube on my road bike (rear wheel, 25 cm tire). I have patched countless inner tubes, including on road bikes. I followed all the steps: found the leak, roughed up the surface of the inner tube around the leak, applied rubber cement, let it dry completely (about five minutes), put on an appropriately sized patch (trimmed down with scissors so that it is not too big), found and removed the cause of the puncture (a bit of glass in the tire).
On the following Monday, I rode my bike to and from work (total 20 km), no problem. Then on Tuesday morning as I was getting ready to ride to work, I discovered that the rear tire was completely flat. I dipped the inner tube in water to find the location of the leak: air was quickly bubbling from the edge of the patch that I had just put on the inner tube two days earlier. I easily removed the patch (it just peeled right off). I thought maybe the repair had failed because I had not made the surface of the inner tube under the patch sufficiently rough before applying the patch, so this time I made sure to make it quite rough. Then I reapplied rubber cement, let it dry for five minutes, and put on a new small patch.
I rode to work that morning (Tuesday), no problem, but then in the evening I discovered my rear wheel was completely flat yet again. Fortunately, there was a bike shop within walking distance. Air was again leaking from the edge of the patch again, but this time the air was bubbling slower than the previous time.
The guy at the bike shop said, as a rule, they do not patch punctured road bike inner tubes (except in emergencies), because the patches sometimes fail due to the narrow width of the inner tube. I had never heard this before (and as I mentioned, I have repaired countless road bike inner tubes), and a subsequent internet search turned up nothing about this, but I guess their experience is worth considering. (They replaced the inner tube.)
My question is, why did my patched inner tube work fine for a few days, and then start to quickly leak air?