diff --git a/changes/ticket40908 b/changes/ticket40908
new file mode 100644
index 0000000000..28cd3f0f36
--- /dev/null
+++ b/changes/ticket40908
@@ -0,0 +1,5 @@
+  o Minor bugfixes (conflux):
+    - Make sure we don't process a closed circuit when packaging data. This lead
+      to a non fatal BUG() spamming logs. Fixes bug 40908; bugfix on
+      0.4.8.1-alpha.
+
diff --git a/changes/ticket40921 b/changes/ticket40921
new file mode 100644
index 0000000000..5818b91864
--- /dev/null
+++ b/changes/ticket40921
@@ -0,0 +1,3 @@
+  o Minor bugfixes (conflux):
+    - Avoid a potential hard assert (crash) when sending a cell on a Conflux
+      set. Fixes bug 40921; bugfix on 0.4.8.1-alpha.
diff --git a/src/core/or/circuitbuild.c b/src/core/or/circuitbuild.c
index d6e022e7fa..dc1912294b 100644
--- a/src/core/or/circuitbuild.c
+++ b/src/core/or/circuitbuild.c
@@ -816,8 +816,10 @@ circuit_deliver_create_cell,(circuit_t *circ,
   circuit_set_n_circid_chan(circ, id, circ->n_chan);
   cell.circ_id = circ->n_circ_id;
 
-  append_cell_to_circuit_queue(circ, circ->n_chan, &cell,
-                               CELL_DIRECTION_OUT, 0);
+  if (append_cell_to_circuit_queue(circ, circ->n_chan, &cell,
+                                   CELL_DIRECTION_OUT, 0) < 0) {
+    return -1;
+  }
 
   if (CIRCUIT_IS_ORIGIN(circ)) {
     /* Update began timestamp for circuits starting their first hop */
diff --git a/src/core/or/conflux.c b/src/core/or/conflux.c
index 677df95067..9025dcffaf 100644
--- a/src/core/or/conflux.c
+++ b/src/core/or/conflux.c
@@ -531,7 +531,10 @@ conflux_note_cell_sent(conflux_t *cfx, circuit_t *circ, uint8_t relay_command)
   }
 
   leg = conflux_get_leg(cfx, circ);
-  tor_assert(leg);
+  if (leg == NULL) {
+    log_fn(LOG_PROTOCOL_WARN, LD_BUG, "No Conflux leg after sending a cell");
+    return;
+  }
 
   leg->last_seq_sent++;
 
diff --git a/src/core/or/conflux_util.c b/src/core/or/conflux_util.c
index 31ab983f8f..4277424ec8 100644
--- a/src/core/or/conflux_util.c
+++ b/src/core/or/conflux_util.c
@@ -33,6 +33,13 @@ int
 circuit_get_package_window(circuit_t *circ,
                            const crypt_path_t *cpath)
 {
+  /* We believe it is possible to get a closed circuit related to the
+   * on_circuit pointer of a connection not being nullified before ending up
+   * here. Else, this can lead to loud bug like experienced in #40908. */
+  if (circ->marked_for_close) {
+    return 0;
+  }
+
   if (circ->conflux) {
     if (CIRCUIT_IS_ORIGIN(circ)) {
       tor_assert_nonfatal(circ->purpose ==
diff --git a/src/core/or/relay.c b/src/core/or/relay.c
index 0cda6e7bf7..5841a56ffa 100644
--- a/src/core/or/relay.c
+++ b/src/core/or/relay.c
@@ -365,14 +365,19 @@ circuit_receive_relay_cell(cell_t *cell, circuit_t *circ,
                                   * we might kill the circ before we relay
                                   * the cells. */
 
-  append_cell_to_circuit_queue(circ, chan, cell, cell_direction, 0);
+  if (append_cell_to_circuit_queue(circ, chan, cell, cell_direction, 0) < 0) {
+    return -END_CIRC_REASON_RESOURCELIMIT;
+  }
   return 0;
 }
 
 /** Package a relay cell from an edge:
  *  - Encrypt it to the right layer
  *  - Append it to the appropriate cell_queue on <b>circ</b>.
- */
+ *
+ * Return 1 if the cell was successfully sent as in queued on the circuit.
+ * Return 0 if the cell needs to be dropped as in ignored.
+ * Return -1 on error for which the circuit should be marked for close. */
 MOCK_IMPL(int,
 circuit_package_relay_cell, (cell_t *cell, circuit_t *circ,
                            cell_direction_t cell_direction,
@@ -430,8 +435,8 @@ circuit_package_relay_cell, (cell_t *cell, circuit_t *circ,
   }
   ++stats_n_relay_cells_relayed;
 
-  append_cell_to_circuit_queue(circ, chan, cell, cell_direction, on_stream);
-  return 0;
+  return append_cell_to_circuit_queue(circ, chan, cell,
+                                      cell_direction, on_stream);
 }
 
 /** If cell's stream_id matches the stream_id of any conn that's
@@ -742,13 +747,24 @@ relay_send_command_from_edge_,(streamid_t stream_id, circuit_t *orig_circ,
     circuit_sent_valid_data(origin_circ, rh.length);
   }
 
-  if (circuit_package_relay_cell(&cell, circ, cell_direction, cpath_layer,
-                                 stream_id, filename, lineno) < 0) {
+  int ret = circuit_package_relay_cell(&cell, circ, cell_direction,
+                                       cpath_layer, stream_id, filename,
+                                       lineno);
+  if (ret < 0) {
     log_warn(LD_BUG,"circuit_package_relay_cell failed. Closing.");
     circuit_mark_for_close(circ, END_CIRC_REASON_INTERNAL);
     return -1;
+  } else if (ret == 0) {
+    /* This means we should drop the cell or that the circuit was already
+     * marked for close. At this point in time, we do NOT close the circuit if
+     * the cell is dropped. It is not the case with arti where each circuit
+     * protocol violation will lead to closing the circuit. */
+    return 0;
   }
 
+  /* At this point, we are certain that the cell was queued on the circuit and
+   * thus will be sent on the wire. */
+
   if (circ->conflux) {
     conflux_note_cell_sent(circ->conflux, circ, relay_command);
   }
@@ -3380,8 +3396,13 @@ relay_consensus_has_changed(const networkstatus_t *ns)
  * The given <b>cell</b> is copied onto the circuit queue so the caller must
  * cleanup the memory.
  *
- * This function is part of the fast path. */
-void
+ * This function is part of the fast path.
+ *
+ * Return 1 if the cell was successfully sent.
+ * Return 0 if the cell can not be sent. The caller MUST NOT close the circuit.
+ * Return -1 indicating an error and that the caller should mark the circuit
+ * for close. */
+int
 append_cell_to_circuit_queue(circuit_t *circ, channel_t *chan,
                              cell_t *cell, cell_direction_t direction,
                              streamid_t fromstream)
@@ -3392,8 +3413,9 @@ append_cell_to_circuit_queue(circuit_t *circ, channel_t *chan,
   int32_t max_queue_size;
   int circ_blocked;
   int exitward;
-  if (circ->marked_for_close)
-    return;
+  if (circ->marked_for_close) {
+    return 0;
+  }
 
   exitward = (direction == CELL_DIRECTION_OUT);
   if (exitward) {
@@ -3423,9 +3445,8 @@ append_cell_to_circuit_queue(circuit_t *circ, channel_t *chan,
            "Closing circuit for safety reasons.",
            (exitward) ? "Outbound" : "Inbound", queue->n,
            max_queue_size);
-    circuit_mark_for_close(circ, END_CIRC_REASON_RESOURCELIMIT);
     stats_n_circ_max_cell_reached++;
-    return;
+    return -1;
   }
 
   /* Very important that we copy to the circuit queue because all calls to
@@ -3436,8 +3457,9 @@ append_cell_to_circuit_queue(circuit_t *circ, channel_t *chan,
   /* Check and run the OOM if needed. */
   if (PREDICT_UNLIKELY(cell_queues_check_size())) {
     /* We ran the OOM handler which might have closed this circuit. */
-    if (circ->marked_for_close)
-      return;
+    if (circ->marked_for_close) {
+      return 0;
+    }
   }
 
   /* If we have too many cells on the circuit, note that it should
@@ -3461,6 +3483,7 @@ append_cell_to_circuit_queue(circuit_t *circ, channel_t *chan,
 
   /* New way: mark this as having waiting cells for the scheduler */
   scheduler_channel_has_waiting_cells(chan);
+  return 1;
 }
 
 /** Append an encoded value of <b>addr</b> to <b>payload_out</b>, which must
diff --git a/src/core/or/relay.h b/src/core/or/relay.h
index 3a1f3b0ae5..12198ae982 100644
--- a/src/core/or/relay.h
+++ b/src/core/or/relay.h
@@ -76,9 +76,9 @@ void cell_queue_append_packed_copy(circuit_t *circ, cell_queue_t *queue,
                                    int exitward, const cell_t *cell,
                                    int wide_circ_ids, int use_stats);
 
-void append_cell_to_circuit_queue(circuit_t *circ, channel_t *chan,
-                                  cell_t *cell, cell_direction_t direction,
-                                  streamid_t fromstream);
+int append_cell_to_circuit_queue(circuit_t *circ, channel_t *chan,
+                                 cell_t *cell, cell_direction_t direction,
+                                 streamid_t fromstream);
 
 void destroy_cell_queue_init(destroy_cell_queue_t *queue);
 void destroy_cell_queue_clear(destroy_cell_queue_t *queue);
diff --git a/src/feature/relay/circuitbuild_relay.c b/src/feature/relay/circuitbuild_relay.c
index 5b1609a1af..ce6cbe6df4 100644
--- a/src/feature/relay/circuitbuild_relay.c
+++ b/src/feature/relay/circuitbuild_relay.c
@@ -579,8 +579,10 @@ onionskin_answer(struct or_circuit_t *circ,
 
   int used_create_fast = (created_cell->cell_type == CELL_CREATED_FAST);
 
-  append_cell_to_circuit_queue(TO_CIRCUIT(circ),
-                               circ->p_chan, &cell, CELL_DIRECTION_IN, 0);
+  if (append_cell_to_circuit_queue(TO_CIRCUIT(circ), circ->p_chan,
+                                   &cell, CELL_DIRECTION_IN, 0) < 0) {
+    return -1;
+  }
   log_debug(LD_CIRC,"Finished sending '%s' cell.",
             used_create_fast ? "created_fast" : "created");