Add regtest block generation error guidance

examples/testing.rst

@@ -39,6 +39,15 @@ Start ``bitcoind`` in regtest mode to create a private block chain.
 
 Generate 101 blocks using a special `RPC <../reference/rpc/index.html>`__ which is only available in regtest mode. This takes less than a second on a generic PC. Because this is a new block chain using Bitcoin’s default rules, the first blocks pay a block reward of 50 bitcoins. Unlike mainnet, in regtest mode only the first 150 blocks pay a reward of 50 bitcoins. However, a block must have 100 confirmations before that reward can be spent, so we generate 101 blocks to get access to the coinbase transaction from block #1.
 
+If block generation fails with an error such as ``CreateNewBlock:
+TestBlockValidity failed: bad-fork-prior-to-checkpoint (code 67)``, make
+sure that both ``bitcoind`` and ``bitcoin-cli`` are using ``-regtest`` and
+that you are not reusing stale regtest chain data from an earlier test. Stop
+``bitcoind``, delete only the ``regtest`` subdirectory of the Bitcoin Core
+configuration directory, restart ``bitcoind -regtest -daemon``, and run the
+matching generation command for your Bitcoin Core version again. Bitcoin Core
+18.0 and later should use ``generatetoaddress`` as shown above.
+
 .. highlight:: bash
 
 ::