Add regtest block generation troubleshooting

examples/testing.rst

@@ -39,6 +39,12 @@ 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)``, check the command and the regtest chain state:
+
+* Bitcoin Core 18.0 and later removed the old ``generate`` RPC. Use ``generatetoaddress`` with an address from the same regtest wallet, as shown above.
+* If you are reusing an old or forked regtest directory, stop Bitcoin Core, remove only the ``regtest`` subdirectory from the Bitcoin Core configuration directory, restart with ``bitcoind -regtest -daemon``, and generate the initial 101 blocks again.
+* Confirm every command is using ``-regtest``. Mixing mainnet, testnet, and regtest data directories can leave ``bitcoin-cli`` talking to the wrong node or wallet.
+
 .. highlight:: bash
 
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