Here's a couple of examples of low quality aftermarket cylinder blocks. Aparently the bimetallization stage of the manufacturing process is more complicated than one might figure. This type of failure is relatively common for cheap Chinese-made cylinder blocks - the bronze layer detaches during the first hour/minute of normal operation at pressures often lower than nominal. Such barrels normally present no visible external deficiencies before the failure.
I once saw a Sauer Danfoss series 90 aftermartket cylinder block detach the bronze layer at 300 bar during the first 30 seconds of the first test run.