A block may be mined with nBits set to the minimum difficulty if its
nTime is set more than six block intervals (15 minutes) after its parent
block.
This is a consensus rule change on testnet that will result in a chain
split (as desired).
The min-difficulty blocks are incompatible with difficulty averaging.
Network difficulty is also now defined as the difficulty the network is
currently working to solve, rather than the last non-min-difficulty block
difficulty.
ca333