The Schwalbe Marathon tyre on my back wheel had been slowly cracking -- both last summer and the year before I had to patch the inner tube where it had abraded against the inner surface of the tyre (leading to a very slow leak), and then tape the tyre.
This morning, I pumped up the back tyre, and wondered if this was happening again. A mile from the office, on the off-road stretch, there was a sudden pop and hiss as the back tyre deflated.
Yes, cracked through by a chunk of flint. Wheel to the nearest bit of paved surface, and put a patch on inner tube and a bigger one on the tyre. That held, enough to get to work, get home, head off the to pharmacy in the next village and back. Now to put on the Super Marathon I acquired in anticipation.