A can't-happen case for one-hop circuits actually can happen
On commit 4dc7b3ca2825741311b6b849ed109053c643ee23, using the controller EXTENDCIRCUIT command to create a new circuit like this:
EXTENDCIRCUIT 0 0744F2AE098BAD9F1A0FEF109C01E621FB6A4600
causes this log message:
Jun 30 12:50:44.000 [info] pathbias_should_count(): Bug: One-hop circuit has length 1. Path state is new. Circuit is a General-purpose client currently doing handshakes. (on Tor 0.2.9.0-alpha-dev 1182ecb3127a6e67)
Jun 30 12:50:44.000 [warn] tor_bug_occurred_(): Bug: src/or/circpathbias.c:362: pathbias_should_count: This line should not have been reached. (Future instances of this warning will be silenced.) (on Tor 0.2.9.0-alpha-dev 1182ecb3127a6e67)
Jun 30 12:50:44.000 [warn] Bug: Line unexpectedly reached at pathbias_should_count at src/or/circpathbias.c:362. Stack trace: (on Tor 0.2.9.0-alpha-dev 1182ecb3127a6e67)
Jun 30 12:50:44.000 [warn] Bug: /home/andrea/tor/test/ticket18640/tor-1182ecb3127a6e67f8ff0c5fc0d28e0a3c25ba4b-post(log_backtrace+0x2f) [0x55f6e9] (on Tor 0.2.9.0-alpha-dev 1182ecb3127a6e67)
Jun 30 12:50:44.000 [warn] Bug: /home/andrea/tor/test/ticket18640/tor-1182ecb3127a6e67f8ff0c5fc0d28e0a3c25ba4b-post(tor_bug_occurred_+0x18e) [0x578780] (on Tor 0.2.9.0-alpha-dev 1182ecb3127a6e67)
Jun 30 12:50:44.000 [warn] Bug: /home/andrea/tor/test/ticket18640/tor-1182ecb3127a6e67f8ff0c5fc0d28e0a3c25ba4b-post() [0x4bcc40] (on Tor 0.2.9.0-alpha-dev 1182ecb3127a6e67)
Jun 30 12:50:44.000 [warn] Bug: /home/andrea/tor/test/ticket18640/tor-1182ecb3127a6e67f8ff0c5fc0d28e0a3c25ba4b-post(pathbias_count_build_attempt+0x23) [0x4bcd8d] (on Tor 0.2.9.0-alpha-dev 1182ecb3127a6e67)
Jun 30 12:50:44.000 [warn] Bug: /home/andrea/tor/test/ticket18640/tor-1182ecb3127a6e67f8ff0c5fc0d28e0a3c25ba4b-post(circuit_finish_handshake+0x1c) [0x4c2ea6] (on Tor 0.2.9.0-alpha-dev 1182ecb3127a6e67)
Jun 30 12:50:44.000 [warn] Bug: /home/andrea/tor/test/ticket18640/tor-1182ecb3127a6e67f8ff0c5fc0d28e0a3c25ba4b-post() [0x4dc1af] (on Tor 0.2.9.0-alpha-dev 1182ecb3127a6e67)
Jun 30 12:50:44.000 [warn] Bug: /home/andrea/tor/test/ticket18640/tor-1182ecb3127a6e67f8ff0c5fc0d28e0a3c25ba4b-post(command_process_cell+0xa2) [0x4db6ab] (on Tor 0.2.9.0-alpha-dev 1182ecb3127a6e67)
Jun 30 12:50:44.000 [warn] Bug: /home/andrea/tor/test/ticket18640/tor-1182ecb3127a6e67f8ff0c5fc0d28e0a3c25ba4b-post(channel_queue_cell+0x207) [0x4b1b32] (on Tor 0.2.9.0-alpha-dev 1182ecb3127a6e67)
Jun 30 12:50:44.000 [warn] Bug: /home/andrea/tor/test/ticket18640/tor-1182ecb3127a6e67f8ff0c5fc0d28e0a3c25ba4b-post(channel_tls_handle_cell+0x2b0) [0x4b79ea] (on Tor 0.2.9.0-alpha-dev 1182ecb3127a6e67)
Jun 30 12:50:44.000 [warn] Bug: /home/andrea/tor/test/ticket18640/tor-1182ecb3127a6e67f8ff0c5fc0d28e0a3c25ba4b-post() [0x50c28d] (on Tor 0.2.9.0-alpha-dev 1182ecb3127a6e67)
Jun 30 12:50:44.000 [warn] Bug: /home/andrea/tor/test/ticket18640/tor-1182ecb3127a6e67f8ff0c5fc0d28e0a3c25ba4b-post(connection_or_process_inbuf+0x141) [0x509038] (on Tor 0.2.9.0-alpha-dev 1182ecb3127a6e67)
Jun 30 12:50:44.000 [warn] Bug: /home/andrea/tor/test/ticket18640/tor-1182ecb3127a6e67f8ff0c5fc0d28e0a3c25ba4b-post() [0x4fd4c7] (on Tor 0.2.9.0-alpha-dev 1182ecb3127a6e67)
Jun 30 12:50:44.000 [warn] Bug: /home/andrea/tor/test/ticket18640/tor-1182ecb3127a6e67f8ff0c5fc0d28e0a3c25ba4b-post() [0x4fac08] (on Tor 0.2.9.0-alpha-dev 1182ecb3127a6e67)
Jun 30 12:50:44.000 [warn] Bug: /home/andrea/tor/test/ticket18640/tor-1182ecb3127a6e67f8ff0c5fc0d28e0a3c25ba4b-post(connection_handle_read+0x1d) [0x4fad32] (on Tor 0.2.9.0-alpha-dev 1182ecb3127a6e67)
Jun 30 12:50:44.000 [warn] Bug: /home/andrea/tor/test/ticket18640/tor-1182ecb3127a6e67f8ff0c5fc0d28e0a3c25ba4b-post() [0x43045c] (on Tor 0.2.9.0-alpha-dev 1182ecb3127a6e67)
Jun 30 12:50:44.000 [warn] Bug: /usr/lib/x86_64-linux-gnu/libevent-2.0.so.5(event_base_loop+0x7fc) [0x7ffff76883dc] (on Tor 0.2.9.0-alpha-dev 1182ecb3127a6e67)
Jun 30 12:50:44.000 [warn] Bug: /home/andrea/tor/test/ticket18640/tor-1182ecb3127a6e67f8ff0c5fc0d28e0a3c25ba4b-post() [0x4335ed] (on Tor 0.2.9.0-alpha-dev 1182ecb3127a6e67)
Jun 30 12:50:44.000 [warn] Bug: /home/andrea/tor/test/ticket18640/tor-1182ecb3127a6e67f8ff0c5fc0d28e0a3c25ba4b-post() [0x433753] (on Tor 0.2.9.0-alpha-dev 1182ecb3127a6e67)
Jun 30 12:50:44.000 [warn] Bug: /home/andrea/tor/test/ticket18640/tor-1182ecb3127a6e67f8ff0c5fc0d28e0a3c25ba4b-post(do_main_loop+0x41b) [0x43352e] (on Tor 0.2.9.0-alpha-dev 1182ecb3127a6e67)
Jun 30 12:50:44.000 [warn] Bug: /home/andrea/tor/test/ticket18640/tor-1182ecb3127a6e67f8ff0c5fc0d28e0a3c25ba4b-post(tor_main+0xfd) [0x43772b] (on Tor 0.2.9.0-alpha-dev 1182ecb3127a6e67)
Jun 30 12:50:44.000 [warn] Bug: /home/andrea/tor/test/ticket18640/tor-1182ecb3127a6e67f8ff0c5fc0d28e0a3c25ba4b-post(main+0x20) [0x42efd6] (on Tor 0.2.9.0-alpha-dev 1182ecb3127a6e67)
Jun 30 12:50:44.000 [warn] Bug: /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf5) [0x7ffff6870b45] (on Tor 0.2.9.0-alpha-dev 1182ecb3127a6e67)
Jun 30 12:50:44.000 [warn] Bug: /home/andrea/tor/test/ticket18640/tor-1182ecb3127a6e67f8ff0c5fc0d28e0a3c25ba4b-post() [0x42eee9] (on Tor 0.2.9.0-alpha-dev 1182ecb3127a6e67)