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With a somewhat recent Git master (commit 2725a88d5e3ec362a4f866f53c26ca20f943eb49):

Oct 09 19:13:41.000 [info] circuit_expire_building(): Abandoning circ [redacted] (state 3:open, purpose 6)
Oct 09 19:13:41.000 [info] internal (high-uptime) circ (length 4): [redacted](open) [redacted](open) [redacted](open) [redacted](open)
Oct 09 19:13:41.000 [err] _circuit_mark_for_close(): Bug: circuitlist.c:1222: _circuit_mark_for_close: Assertion ocirc->rend_data failed; aborting.

(CIRCUIT_PURPOSE_C_INTRODUCING == 6)

The assertion occurred in the following chunk of code, added to help fix #3825 (moved):

 } else if (circ->purpose == CIRCUIT_PURPOSE_C_INTRODUCING &&
             reason != END_STREAM_REASON_TIMEOUT) {
    origin_circuit_t *ocirc = TO_ORIGIN_CIRCUIT(circ);
    tor_assert(ocirc->build_state->chosen_exit);
    tor_assert(ocirc->rend_data);
    log_info(LD_REND, "Failed intro circ %s to %s "
             "(building circuit to intro point). "
             "Marking intro point as possibly unreachable.",
             safe_str_client(ocirc->rend_data->onion_address),
           safe_str_client(build_state_get_exit_nickname(ocirc->build_state)));
    rend_client_report_intro_point_failure(ocirc->build_state->chosen_exit,
                                           ocirc->rend_data,
                                           INTRO_POINT_FAILURE_UNREACHABLE);

But we should never have a circuit with that purpose with its rend_data field unset.