All right folks, here's one more. Start from scratch on testnet. Set gen to 1. Result:

ERROR: ContextualCheckBlockHeader : forked chain older than last checkpoint (height X(5, 17, etc...))
terminate called after throwing an instance of 'std::runtime_error'
what(): CreateNewBlock() : TestBlockValidity failed

Pretty self-explanatory.

main.cpp ln: 2597

// Don't accept any forks from the main chain prior to last checkpoint
CBlockIndex* pcheckpoint = Checkpoints::GetLastCheckpoint();
if (pcheckpoint && nHeight < pcheckpoint->nHeight)

2597: return state.DoS(100, error("%s : forked chain older than last checkpoint (height %d)", func, nHeight));

Call stack:

0 ContextualCheckBlockHeader main.cpp 2597
1 TestBlockValidity main.cpp 2774
2 CreateNewBlock miner.cpp 335
3 CreateNewBlockWithKey miner.cpp 411
4 BitcoinMiner miner.cpp 468
5 boost::_bi::list1boost::_bi::value<CWallet* >::operator()<void ()(CWallet), boost::_bi::list0> bind.hpp 253
6 boost::_bi::bind_t<void, void ()(CWallet), boost::_bi::list1<boost::_bi::value<CWallet*> > >::operator() bind_template.hpp 20
7 boost::detail::thread_data<boost::_bi::bind_t<void, void ()(CWallet), boost::_bi::list1<boost::_bi::value<CWallet*> > > >::run thread.hpp 117