First observed in 13w43a, SSP.
Cobblestone generation seems broken. Cannot determine if the cause is in the generation system, or some problem with water updating incorrectly.
Cobble generation only appears to work if both the lava and the water are both flowing (updating) at the instant of contact.
Steps to reproduction:
1) Create lava source, and let it flow until it stops.
2) Introduce a water source 2 blocks from the end of the lava flow
Occasionally, a cobblestone block WILL be generated, but as soon as the cobble block is broken, or intervening block is placed somewhere in the stream and then removed, the above behavior will resume.
In the pictures below, the pic time-stamped at 7.32.45, the water blocks have stopped updating themselves and will not flow any further. When the piston moves backwards, the water remains in place, as if it is still touching a solid block. The lava then moves forward and touches the water, but no cobble is created