Seen in the wild:
2020-06-09T02:48:46.724000Z TestFramework (INFO): Put txs back into mempool of node 1 (not node 0)
29802020-06-09T02:48:47.052000Z TestFramework (ERROR): Assertion failed
...
gnu/test/functional/test_framework/test_node.py", line 351, in is_node_stopped
2995 assert return_code == 0, self._node_msg(
2996AssertionError: [node 1] Node returned non-zero exit code (66) when stopping
...
https://travis-ci.org/github/bitcoin/bitcoin/jobs/696292300#L2968